iPhone Dev team blog announced that "the ultrasn0w carrier unlock now supports the iPhone4! You'll find version 1.0-1 of ultrasn0w in Cydia on jailbroken devices." "Version 1.0-1 of ultrasn0w works for: iPhone4 baseband 01.59, and 3G/3GS basebands 04.26.08, 05.11.07, 05.12.01 and 05.13.04. If ultrasn0w doesn't show when you search Cydia, add the repo: repo666.ultrasn0w.com."
"Jailbreaking is the act of freeing the iPhone from the restrictions that apple's default settings provide. It allows users to install custom software from a wide variety of different places without the need to use the iTunes App Store."