I can confirm that the AT&T SIM swap method works for Tmobile users.

I have a Tmobile account, and was able to successfully use an AT&T SIM from a co-worker to get the Nokia Messaging option to show up when adding a new account.
After I set up Nokia Messaging, I switched back to my Tmobile SIM, and it's now working without problems.

Love being able to receive the emails almost immediately.
It's great to have IMAP access and all, but waiting 2-5 minutes for my gmail and work email accounts to refresh was a joke.

Nokia Messaging is very good, almost on par with my old Blackberry service.