In a recent interview, the CEO of French Telecom/Orange, Stephane Richard, dropped some hits towards a "smaller and thinner" iPhone 5 with a different SIM card design. Instead of the traditional method, Apple has apparently agreed with wireless carriers to support a smaller and more lightweight SIM for the next iPhone.
Practically, this smaller SIM card would make the iPhone thinner. There is also the possibility that such a SIM card could work in tandem with the rumored 'global device' that Apple could release this year.