Actually, you can just buy a GoPhone card - the SIM pops out of the card and right into your iPhone. That and a $99 refurb iPhone is what I use for app development.
FYI - This works fine if the phone always lives at your desk on WiFi, but take it anywhere and the data access will chew through your prepaid card minutes at lightning speed.
Yes, but one caveat. All the new retail packaged GoPhone's are SIM-locked for 6 months to the device it came with.
Best to stop at an AT&T store and tell them you want to sign up. They'll give you a SIM (use any crap unlocked phone) then you can move it to your iPhone later.