iOS 4.1 has been released for iPhone 3/3G/4 and iPod Touch 2G/3G/4. If you haven’t upgraded yet, check iTunes again or follow our download links in this post. But for those who use an unlocked or jailbroken iPhone or iPod Touch, you have been warned to stay away from this upgrade until there are tools available for iOS 4.1
The iPhone Dev Team has given out the warning on their site:
This time of year there are lots of new iPhone owners, and not everybody knows that accepting new iOS updates is the surest way to lose your jailbreak and/or unlock. While those of you who have Cydia or TinyUmbrella backups of your FW hashes will always be able to get back to 4.0.1 if you make this mistake, this doesn’t hold for unlockers. There’s currently no known way to revert your baseband — if you update your baseband you’ll lose the ultrasn0w unlock, possible forever.
Please stay away from this 4.1 release until a safe jailbreak procedure (which also preserves ultrasn0w) is developed and released.
Make sure you’ve backed up SHSH blobs before you upgrade to iOS 4.1 unless you’re using ultrasn0w, we recommend you don’t upgrade at all because there’s no known method yet to downgrade your baseband from iOS 4.1