Shortly after the iPad announced that it only took Micro-SIMs I commented it looks like you trim down a normal SIM with scissors, and have it fit into a Micro-SIM slot.
Looks like I was right, and TechRadar has a how-to guide for the SIM to Micro-SIM conversion. BGR has a few images of AT&T's new combo-SIMs, and although the AT&T Micro-SIM is a one-way street, MicroSim Shop sells a Micro-SIM (Mini-UICC) to SIM adapter for €5.99.
MicroSim Shop also has a guide for cutting a SIM down to size, and some shots of the final product being used in one of their adapters.





