Micro SIMs rather than the typical Mini SIMs. We've gotten a picture of just such a module (see above), and you can plainly see it's not a hybrid -- it can't be snapped out as a Mini SIM, meaning that unless you've got an adapter lying around, you need to head into the store to get it swapped out for the right kind. The only consumer devices out there in any measurable quantity right now that make use of Micro SIMs are, of course, the iPad and iPhone 4 -- and when you pair this up with the fact that T-Mobile stores are starting to sell iPhone cables right now, the situation is starting to get wacky. Now, we still want to stop short of suggesting this means T-Mobile is getting either the iPad or iPhone any time soon; after all, Apple would have to provide a new hardware variant that supports AWS 3G, and it's totally reasonable to think the carrier could be offering other phones in the near future that'll use Micro SIMs. We do love us a good conspiracy theory, though.
[Thanks to everyone who sent this in]
Update: We're hearing it's for the Galaxy Tab. Mystery solved!
Update 2: Our own Richard Lai notes that the European version of the Galaxy Tab uses a Mini SIM, but we're hearing some pretty strong evidence that T-Mobile's will be Micro for whatever reason. That'd be an odd thing to switch up, yes -- but if any major manufacturer could quickly churn out umpteen different versions of a device with moderately different internals, it'd be Samsung. Guess we'll know soon enough.