If you're talking about the newest SIM form factor, it's called Micro-SIM. The one you're familiar with is the Mini-SIM.i see the small sim and it's something that tmobile doesn't offer (i just called to make sure) will this new iphone only take a mini sim? if so that leaves a lot of us SOL!
Official carriers for the next-generation iPhone would offer the new Micro-SIM, just as they will for the iPad. Or you could use a pair of scissors or utility knife and hack away at the plastic (watch those fingers).