Question about risk of selling iPhone

[r A p h]

Hey, I just bought the new 3GS, and am thinking about selling my current iPhone 3G. My 3G is tied to my AT&T 2 year contract (or at least the SIM card is), and I'm wondering if there is a risk for me or for the buyer when I sell my phone? Will anyone be able to put in their AT&T SIM card and not be forced (and unable) to activate it and have to take out an AT&T contract? What if they restore the phone, will they have to re-activate the phone and might run into problems? Thanks!