Assuming you use the iPhone with AT&T, you might be able to get a month-by-month contract, but AT&T will want to sell you a data plan unless you tell them you have a "non-smart phone" and then switch the SIMM after you leave the store.
I used my first iPhone 1G/2G with T-Mobile for 18 months this way (jailbroken and unlocked, of course). Not having data everywhere sucked, but it worked like an iPod touch with phone capabilities when I was near wireless I could access.
I think the 3G can be unlocked, but the 3GS is still not quite there (although the latest is that someone has figured out how to unlock the 3GS AND the upcoming iPad).