T-Mobile USA Offers Micro SIM: Perfect For Jailbroken iPhone Handsets
"Already have a GSM phone or iPhone that you need to activate—or have an unlocked GSM phone you want to use on the T-Mobile network? Just get a Micro SIM card and choose a plan. Slip the ready-to-use Micro SIM Card into your phone and you’re ready to go."
Notice the careful usage of "iPhone" in here? The moral of the story seems pretty clear: if you need a Micro SIM to more easily use your unlocked iPhone 4 on T-Mobile's network, they're more than happy to help you out.