According to many theists who believe in intercessory prayer, as long as you have strong enough faith, your wish is god's command. Apparently, to such people, their god has been demoted from universal creator to their own personal magic genie. In fact those who follow the Christian bible, there is a line contained within that states the power of faith. It is the often over quoted Mathew 17:20:
"And He said to them, "Because of the littleness of your faith; for truly I say to you, if you have faith the size of a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible to you."
Really, that's all it takes? There must be mountains moving all over the place to the delight of Christians! Wait, wouldn't that cause massive tectonic upheaval resulting in devastating earthquakes and tsunamis around the globe on a frequent basis... So next time you hear a fundamentalist say that such a natural disaster was the result of god's anger at the 'evils' of homosexuals or whatever has got his ephemeral panties in a bind today, know that it just the Christian's way of covering their own tracks at moving around all that earth.
But in all seriousness, if so little faith is required for a miracle (and I define a miracle as an event occurring that no natural process could possibly explain. And no, your aunt who survived cancer was not a miracle but the result of doctors and researchers working tirelessly to save the lives of their fellow humans, thank them), then why do we not see them all over the bloody place? With such a tiny amount of faith being required to do a miracle, we should be so deep in them to not be able to ignore their statistical significance. But guess what, we don't. We don't see a single one. So either all Christians (and the members of other faiths who have similar doctrines about intercessory prayer and faith) are faking their piety or the whole concept is utterly devoid of fact.
About this time, someone from the background who thinks they are clever (thinks being the key word here) pipes up with "But god does answer prayers, he (And it is almost always a he, notice that? Why does god need a gender, is there a breeding colony of them somewhere? Is that why 'he' has been ignoring us? Has he has been to busy getting it on with Aphrodite?) just usually says no". About this time I have to do the stereotypical cartoonish blink a few times to let this new 'argument' process. To which I respond, "So your god just happens to answer prayers in exactly the same ratio as one would expect from a random system that is not ruled by any deity at all?" To which I hear "Yes! god works in mysterious ways, doesn't he?". About this time I have to run the conversation back through my head. Not because I am confused, but because I am desperately hoping I had missed something, as the alternative is far to painful for my mind to grasp. "So you are telling me" I will reply in a slow manner to make sure no words are lost "that your god is all powerful, yet he, for some mysterious reason, decides to ignore this and work at a rate no better then random chance?". If this question is answered in the affirmative, as it almost always is, my hope for my species dies just a little bit more.
Prayer has another serious problem about it. If your deity is omnipotent and omnipresent, why would it need some little peon like yourself to tell it what to do? Is the deity that incompetent that it needs its own minions to keep an eye out for it? And how exactly would this be a deity that is worth worshiping in the first place? This little bit of self contradiction shows that most theists really do not believe their deity is all knowing. In fact it makes it seem like they see their god as little more then a tool at their disposal.
But let's be fair. Let's suppose that they truly believe that unanswered prayers are the way their deity says no. In that case, can all the nut jobs who are protesting outside woman's health clinics, who are against the 'evils' of homosexuality, trying to get their myths inserted into school curriculum, who wish to stop president Obama from doing the, ironically, Jesus like thing of helping out his fellow man through universal health care, or whatever insane thing you hate today, could you stop and realize that all your prayers have thus far done nothing at all. If you so believe in your deity's existance, perhaps this is his way of telling you that you are wrong and that he likes these ideas. After all, if they were so evil, wouldn't your god step in and do something?