The DevTeam and others have been checking through the iPhone 4, and have noticed the baseband runs a completely different OS than on previous models. The iPhone 3G and 3G[S] both ran Nucleus OS on the PMB8878 baseband CPU (aka XGold 608), where the iPhone 2G ran the PMB8876 (aka S-Gold 2). iPhone 4 uses the PMB9800, or X-Gold 618 - running a separate OS - ThreadX, an RTOS by ExpressLogic.
What's all that mean to you?
Basically, once the iPhone 4 is jailbroken, the unlock won't be an immediate release. iOS4 is already un-lockable using ultrasn0w 0.93 on the earlier iPhone models, but the payload will need to be rewritten for iPhone 4 once an exploit is discovered in the new baseband OS. Hopefully the port itself will have introduced some. No way to estimate any sort of timeline until further investigating is done.