According to a post at Neowin, an exploit was found in the A4 processor, which means that the iPhone 4 is jailbreakable for life, till a new hardware revision comes:
http://www.neowin.net/news/new-exploit-found-in-iphone-hardware-makes-it-jailbreakable-for-life
http://www.neowin.net/news/new-exploit-found-in-iphone-hardware-makes-it-jailbreakable-for-life