Infamous DVD Jon cannot leave an encryption unhacked. On his web site "So Sue Me" he published a little tool to activate the iPhone without registering with AT&T over iTunes.

The activation of the iPhone still does not give you an unlocked iPhone. After the fake activation you can use the music player and the WiFi connection of the iPhone.
As reported the iPhone SIM card is easy to remove, but using another SIM card is not possible. Looking at the pace that hackers succeed in dismantling the iPhone security mechanisms unlocked iPhones may be available soon.