All I know is that 2.0 has been jailbroke. Upgrading to the 2.0 software won't give you true gps and 3g speed, only buying a iphone 3g do you get that.

The app store and push email(via exchange) is the only plus side of the 2.0 software, but most of the apps don't work very well yet.

I don't know for a fact if anyone in your situation has upgraded and still been able to use t-mobile. I would try to find someone that upgraded without trouble using your phone and service before I made the leap.

If your phone is already jailbroke, you have access to apps that are in some ways better than what you can get in the appstore. The jailbreak last.fm audioscrobber app is superior to the one in the legit appStore.

If you don't need push email and exchange calendars, there isn't much you can't get already with a jailbroke phone.