I'm assuming that your iPhone is unlocked. You didn't mention either way, but if you're asking about non AT&T SIM card I'll assume that it is.
You should be able to use ALMOST any GPRS SIM car. Not necessarily any, because who really know which will and which won't. I know for a fact that any AT&T sim will work. Outside of that, trial and error. Supposedly, any will.
Let us know what you find.