Ok, so here is the question: If today I choose to pray that event E that happened yesterday would not have happened, (a) is it possible that God answer this prayer and (b) is it plausible that God would answer this prayer. I'm calling this "Retroactive Impetratory Prayer" (hereafter, RIP).
I've come to the conclusion that (a) depends on which theological view to you hold to with regard to divine providence. Let's start there.
Suppose I am an Open Theist approaching this question. The Open Theist almost certainly has to answer in the negative. On Open Theism, God does not have complete, definite foreknowledge. He can be 99.999% certain of most things and even 100% certain of some things, but most things are probably in the "extremely well-informed guess" category rather than the "absolute certainty" category. If this is the case, then there is no way that God could really justify answering a prayer that He doesn't know with absolute certainty will be prayed. There's no way, then, that He actually would answer a prayer prior to its being prayed since it's still possible that the prayer won't be prayed. This could change once all the necessary and sufficient conditions are met for a given prayer's being prayed. Once that happens, I would think that answering the prayer prior to its instantiation would be fair game for God on Open Theism as well.
The Augustinian/Calvinist (A/C) (determinism or compatibilism, take your pick) view would also have to answer in the negative. Strictly speaking, the past happened the way it did because God intentionally caused it to be that way in every case. For God to answer a prayer prior to its being prayed, God would have to essentially admit that His plan was flawed to start with and then change it. Of course, some of you A/C folks out there may disagree with my interpretation here, and if so I'd appreciate hearing about it. One reason you may want to disagree is that this exact same reasoning can be used against the possibility of impetratory prayer of any kind on the A/C view, which is something I bet a lot of folks will want to avoid. And, incidentally, this is yet another great reason not to hold the A/C view. Just sayin'.
The Molinist, it seems to me, is the only one who can really entertain the possibility of RIP being able to work on any consistent basis. First, on Molinism prayer definitely has the potential to impetrate. That is, God's decisions in the world are definitely affected by prayer. Second, God knows prior to the creation of the world exactly what prayers will be prayed at what time and by whom. Third, it seems to me to be possible, if not probable, that God would be willing to take those future prayers into account when guiding events prior to the prayer. So I think that the RIP issue is one that Molinists are (mostly) unique in having to address. So there is my answer to (a). (Brief aside: I'm not really sure if it's possible that a Molinist hold to a B-theory of time. Haven't thought about it too much. If one were to do so, there may be some issues there as well...which I haven't bothered considering. For now, though, I'm going to assume an A-theory of time and move on.)
Now, if a Molinist answers affirmatively to (a), then what of (b)? Well, I think first we need to recognize that on the Molinist account of prayer, God makes the decision of whether or not to answer a prayer prior to the creation of the world, so strictly speaking the time at which a prayer is prayed shouldn't affect His ability or willingness to answer it. The problems really come when we conceptualize the prayer. Say E happened yesterday and it displeased me. Now let's say that I choose today to pray that E never happened. Well, if I know I pray the prayer in the first place I already know that it went unanswered. If there is but one timeline (and I think it's pretty reasonable to think this is the case, especially on the Christian worldview), it follows that events I have already experienced cannot be changed. Those events would have never have been able to take place were my prayer answered retroactively (i.e. prior to my prayer having been prayed). So, then, if I believe in the possibility of RIP, it follows that if I attempt a retroactive prayer I'm already guaranteed that the prayer was not answered.
But this is not the end of the issue! Far from it. No, no. This is right where it gets interesting. See, the mere fact that my praying for retroactive answers guarantees that those retroactive answers didn't happen does not mean that such prayers are completely superfluous! God may actually answer some retroactive prayers, it's just that the believer who would have done the praying would never know it. However, in order for there to be the possibility of such a prayer being prayed, the believer must actually get in the habit of praying such prayers. Example:
Suppose there are two separate and unrelated events E1 & E2. Believer B dislikes both and will pray that both did not happen in the past (these prayers will be P1 & P2, respectively). God decides logically prior to creation whether or not he will answer either prayer. Say God decides to answer P2 but not P1, so that E2 would not have happened, but E1 still would. B would still have to have a mindset that may cause her to pray both P1 & P2 in order for P2 to be answered. Likely, then, prayers of the P1 variety would still have to be prayed (even though B knows the prayers are superfluous) in order to guarantee that prayers of the P2 variety would also have been prayed (and thus not have taken place). Otherwise there is no possibility that God know via middle knowledge that P2 would be prayed (since it would not, in fact, be prayed), and thus He would not prevent E2 from obtaining.
So the answer to (b) above would be: no. It is impossible that God answer the retroactive request you are currently praying for. However, the Molinist can (and maybe even should) believe that such prayers may nevertheless be necessary in order for God to answer other retroactive prayers that she will never actually pray.
I really look forward to some thoughts on this from any of you theology dabblers out there!
**Update**
In a discussion I had on this last night with my roommate, I found that framing the issue in more explicitly Molinist terms was helpful.
Molinism, roughly, is the belief that God obtains His foreknowledge from the combination of three logical moments. Moment 1: natural knowledge (knowledge of all necessary truths). Moment 2: middle knowledge (knowledge of the truth value of all counterfactuals). Next comes the decision to create the world. Knowledge from moments 1 and 2 are then combined with the knowledge of which world was created, which gives God moment 3: Free knowledge (complete, definite foreknowledge of all circumstances that will obtain in the world and all free choices that will be made by creatures in those circumstances).
Consider the example I gave above in Molinist terms:
Prior to creation, God is presented with 2 counterfactuals:
C1. Given E1, B would freely choose P1.
C2. Given E2, B would freely choose P2.
Assuming both counterfactuals are true, God then weighs whether or not He plans to answer either P1 or P2 (or both) and chooses (still prior to creation) to answer P2, but not P1.
E2 now will never obtain. B will only ever pray P1 and B will have no idea that E2 (and thus P2) was ever even a possibility. B will certainly have no idea that God answered P2 without it ever happening.
However, the truth value of counterfactuals is determined by the free creature. So, the truth values of both C1 and of C2 are in the hands of B. It just so happens that God knows what B would choose given each circumstance. So if B does not freely choose to pray retroactive prayers sometimes (already knowing that the prayer will not be answered), then it is a virtual guarantee that the truth values of both C1 and C2 would be false, causing God to not answer P2.
