I had so much trouble finding the answer to this, so I hope when people google search this question that they'll find this blog and thank me!
Gevey sim works with T-mobile on a TETHERED jailbroken iPhone 4 on iOS 4.3.5 and firmware 04.10.01 first try. Don't reboot the phone: just put in your gevey sim (got mine off of eBay for $11 from a guy who sold thousands and guaranteed that it'd work or your money back--it's one where you have to cut a corner off to fit), and wait for the welcome screen and follow the directions. SMS text messaging, phone calls and everything works. I haven't tried facetime yet, but I don't really care about that.
Anyway, there you go, T-mobile iPhone users.