I just bought a 2.0 unlocked and jailbroke 2g iphone and am using it with t-mobile. Everything works great except for the appstore and itunes. I went online and set up a account at itunes and then tried logging in with my phone and it says "invalid password". I did a search and couldnt really find anything...
The iphone maybe synced with another Itunes account. You'll probably need to do a "Factory Restore", set it up as a new iphone, and then Re-Jailbreak it. It will be a pain so be prepared to invest an hour or two to get it to the point that you have it now.