when you are unable to load it, search the forums for vendor.xml and follow those instructions to delete the file then load the OS software. the reason you'd want to use the current carrier software (T-Mobile) rather than the original carrier software (Cingular) is that TMO *could* have made some tweaks and additions to the code that would fix issues related to their network. in most cases, this point really won't matter one way or the other, but you never know... you can read through the Cingular release notes and notice some of these area-specific fixes (I think Oakland, CA or something was mentioned in one of them).
In the beginning the Universe was created. This has made a lot of people very angry and is widely regarded as a bad move.