Thanks to a tip off from one of our Android Forums posters, Superatim, U.S. users can download and install the already released UK version of Android OS 1.5 (cupcake). You will be giving up some of the U.S. features of 1.5, but will gain them back once the U.S. version of 1.5 is released and made available for manual upgrades. If you’re ready to take the U.K. plunge head on over to the T-Mobile Forums and follow the directions provided. And no, root doesn’t seem to be required for this.
Just from reading the original post, it seems the only things that are missing are Voice Search and the Amazon mp3 store.
Update: Seems a few users have been reporting that their contacts are gone upon initial boot; however they seem to return after ~30 minutes. Let the sync do its thing! This will probably hold true for the calendar as well, but that is only speculation.