> If God was not all-knowing, he would not know that I praise him or thank him without me actually telling him. But God is, so he knows I'm thankful for what he has done without me saying "Hey, thanks!"

Thanking someone and being thankful are two different things that illustrate the active vs. passive distinction that has come up in other posts. If the purpose of thanking God in prayer were simply to "inform" God that some thankfulness was going on then, indeed, an omniscient God wouldn't have much use for such a prayer.

The point is that this kind of passiveness doesn't do a single thing to foster a relationship between one and God, which is the true purpose of prayer. Koalamom's post on the subject says it better than I could. Beyond the relationship factor, passively being thankful has a tendency to lead toward taking things for granted. The deliberate act of thanking God keeps the spirit of thankfulness foremost in our minds.