Disposable prepaid SIM cards should provide some anonymity until folks are required to register them.
Originally Posted by S-Man
Nope, this won't work with a stock iPhone 4. To activate an iPhone 4 in this country, you must assign it to an AT&T account. Once you do that, it's locked to the AT&T account and won't work with a prepaid SIM. Not only that, the iPhone 4 uses the micro-sim format, which is not supported by all of the prepaid options.
Yes, you can (easily) jailbreak your iPhone 4 and, if you do, you can unlock it AND use Facetime over 3G. However, most people are too lazy or too worried about voiding their warranty to go this route.