As someone who bought an iPhone independently (via Craigslist), while already under a pre-existing (non-iPhone) contract, I can attest that what Big9 and Flying said is true. Thought I could "get away" with not paying for the data plan, but alas U can't really. AT&T *will* charge U per MB for any data (even received emails), which adds up $$ REAL fast. Plus they will start sending U annoying, daily text messages warning U, "U must be on an iPhone data plan to use data on iPhone". Just not worth the hassle and $ to fight this, IMO.
One thing U can do, to avoid signing a new 2-year contract, is get an amenable ATT rep to set U up with iPhone data plan under ur current contract. Try contacting someone like Blakejackson (on TIPB) who can do this.
One alternative is to jailbreak/unlock ur iPhone & go to t-mobile. U won't get 3G Data speeds, but their talk/data plans are cheaper than ATT's so u'll def save $$ overall.
@ Grkboy, ur alternative sounds interesting, but it sounds like U can only get data via WiFi with the BossPrefs jailbreak hack/app? And does this screw with ur ability to make/receive phone calls??