How can I tell if I have purchased an app before I try to download it?
(I so wish iOS5 would hurry up already)
Load up the App Store. Go to the Updates section, then click on the Purchased listing at the top.
Go to the iTunes Store home screen. At the top right you'll see a list of "Quick Links." Click on the Purchased listing.
Note: Additionally, all listings in iOS will be shown as "install" rather than feature the price, indicating that you have already purchased the app. In iTunes, that is also true, where the price will be replaced by "download."