iPhones are out of stock at most Apple stores. It is predicted that a 3G iPhone will be released very soon. The current iPhone models use AT&T's Edge network. 3G is a faster standard, and will make browsing the internet over AT&T's network much faster. 3G does deplete the battery quicker, and in current beta builds of the next software release, there is an option to turn it off. This option will almost definitely carry into the final release. AT&T offers special plans for the iPhone. All of these come with unlimited data access. Here is the plan list:
I suggest waiting until the next revision of the iPhone comes out. It will include 3G, and probably several other additions.