IPhone 4s on T-Mobile Jailbreak?

I'm thinking about buying an iPhone 4s locked to AT&T. If I Jailbreak it, can I use it for T-Mobile? I had a problem initially with my iPhone 4 where the baseband had not yet been supported when I bought it and I'd like to avoid that problem.

Nope, jailbreaking only adds cydia app.
you would need to get it unlocked to use on tmobile.