no the nexus s 4g doesn't have a sim card slot nor a compatible radio. For a cool $600 you can buy an AT&T compatible Nexus S from Negri electronics. Do not try and buy a T-Mo NS and use it on AT&T as you will not get 3G speeds because of incompatible radio bands.
No problem. Yeah if your good with edge then you can pick up a NS for T-Mo relatively cheap on craigslist. Keep in mind, an AT&T compatible NS was recently spotted on the Samsung website which has rumors swirling. It's long been said the AT&T NS is coming.
If you can wait it out you may be able to pick one up for $100ish by signing a contract with AT&T plus you get the benefit of 3G