http://thefirmwareumbrella.blogspot.com/ This is straight from planetbeing himself, the guy who has the first carrier unlocked iPhone 4. Backup your SHSHs now because in the future, Apple will cease to allow restoring to this firmware if you accidentally upgrade for any reason. EDIT ON JULY 22ND, 2010. According to comex, you can STILL back up your SHSH blobs for 4.0.1. So DO IT NOW in case you missed out on backing up 4.0 blobs.