For post paid service the only one that I know of in the US is Cingular. You will not be eligible for any promotional rates. You will also of course pay the standard activation fee (around $35 IIRC.)
You will also likely need to pass a US credit check or if that's not possible they may ask for a substantial deposit on the account.
If that's not possible you may be relegated to using prepaid service.
There's also a newer service called Consumer Cellularwhich will give you monthly service on the old AT&T TDMA network. You can get a free phone to join. My only comment on them is their rates don't appear to be anywhere as good as most of the majors such as cingular, T-Mobile, Sprint PCS, etc., but if you just want service without a contract that's an option. I don't know what their requirements are regarding whether you need to have a credit check with them.