Just thought I'd report that it is possible. I spent a few weeks figuring this out since there's really not a detailed how-to out there for 3G/T-Mobile/iOS 4.0 official release.

You need to have iOS4 and baseband 05.11.07 (Or an older baseband, 04.28.08?) in order for this to work.  If you have an incompatible baseband, you may be able to downgrade to the right baseband

* Jailbreak with Pwnagetool
* Backup your SHSH blob files for 3.1.3 to the Cydia server
* Build a Jailbroken iOS4 bundle with Pwnagetool
* Update via iTunes using Command+Shift+Restore and your jailbreaky iOS4 install
* Disable 3G, plug in T-Mobile SIM card
* Open Cydia, add "repo666.blackra1n.org"
* Search for "Ultrasn0w 0.9.3"
* Install package, reboot

Congrats, successfully unlocked iOS 4.0.

Did this a week ago.
See my post at the bottom of this thread:

http://www.ar15.com/forums/topic.html?b=1&f=5&t=1065806&light=

Still can't run 3G speeds on T-mo because of the frequency of the radio correct?

Correct.