Well, this is... strange. Or, is it? T-Mobile USA has been somewhat open in the past about the fact that many iPhone users unlock / jailbreak those devices and hop onto a T-Mob plan (albeit with EDGE speeds when it comes to data), but this is something else entirely. The carrier is now offering up a free Micro SIM card, which the iPhone 4 requires for use. Why? Let's allow T-Mob to speak for themselves:

"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.