My friend has used SuperSim (http://www.nowgsm.com/supersim.htm). It copies the contents of your SIM (up to 16 SIMs, in fact) to the SuperSIM card. He has used it to copy a Telstra SIM (from Australia) and a T-Mobile SIM (from the USA). When he puts the SuperSIM in the cell phone it adds another menu item that allows him to swap from one "sim" to another. It seems to work OK. I've just ordered one myself.

Hope this helps,

Glenn