Preview at WWDC likely in June, followed by September launch.
iPhone 4 Unlocking Tool Now Available
The first iPhone 4 unlocking tool has been released. Version 1.0-1 of ultrasn0w provides iPhone 4, 3G and 3GS owners the ability to carrier unlock their iPhone to work across multiple providers.
While in some countries customers may be able to buy unlocked iPhones, most iPhones come locked to the specific mobile carrier. The process of "unlocking" the iPhone allows you to use the same phone across other GSM mobile phone carriers. In the U.S., however, options for alternative carriers are limited as T-Mobile is the only other GSM-based carrier and they do not even support the iPhone's 3G data network. In order to unlock your iPhone, you must first be Jailbroken.
Users should, of course, proceed at their own risk, as none of these procedures are sanctioned by Apple who warns that any issues caused by these procedures could violate your warranty.