Apple has finally released iOS 5.1, but if you want to save your earlier jailbreaks, you are highly recommended to stay away from the firmware update.
According to the iPhone Dev Team, with A5 devices like iPad 2 and iPhone 4S, it's not possible at present to downgrade to any older iOS 5.x firmware, even if the SHSH blobs (Signature HaSH blobs) are saved. However, users of iPad 2 can downgrade back to iOS 4.x as long as they have saved SHSH blobs for it.
Only non-A5 devices like the iPhone 4, 3GS, the original iPad and the iPod touch can be downgraded back to both iOS 5.0.1 (custom firmware only) and iOS 4.x providing the users of the devices have SHSH blobs and APTicket saved for the firmware they are downgrading to.
That said, here's some more good news for those with older A4 devices. Thanks to the earlier limera1n exploit, users with devices earlier than the iPad2 can test the 5.1 jailbreak using Redsn0w 0.9.10b6. Before going deep into the process of jailbreak, here's what the iPhone Dev Team needs you to be aware of:
-This is a tethered 5.1 jailbreak for non-A5 devices (That means the jailbreak does not work on the iPad 2 or iPhone 4S). You'll need to use redsn0w to Just Boot your device every time it power cycles, otherwise jailbreak apps won't work (neither will Safari).
-If you use ultrasn0w for your carrier unlock, be sure to use a custom IPSW to get to 5.1 first! Don't ever restore to a stock Apple IPSW! Use redsn0w's Custom IPSW button to create a NO_BB_* version of the 5.1 IPSW and restore to that instead of the stock one. (That option is available only to 3GS and iPhone4-GSM owners.) ultrasn0w itself will be updated for 5.1 in the next few days (same baseband support, not 5.1's baseband).
-If you're lucky enough to have an old-bootrom 3GS, this jailbreak is actually untethered (redsn0w will figure that part out automatically).
-While we were at it, we added @pod2g's steaks4uce exploit to support MC models of the iPod touch 2G (whose last firmware was 4.2.1). So now redsn0w will auto-detect and jailbreak both MB and MC versions of that older device.
-iBooks won't work until a future update of redsn0w.
How to Jailbreak iOS 5.1 Using Redsn0w 0.9.10b6
Step 2: Place the device into DFU mode.
Step 3: After the device is connected to the computer, start Redsn0w and click jailbreak.
Step 4: For some time, it will go through the process of finding the necessary firmware. After that it will ask you to install Cydia. Select the Install Cydia box and click Next.
Step 5: A lot of scrolling text will appear on the device's screen and after some time you will get back to the home screen. There will be a white Cydia icon, which means that you have to perform a tethered boot.
Step 6: Once again, place your device back into DFU mode.
Step 7: Go back to Redsn0w and select Extras, then Just Boot in order to boot your device tethered.
Step 8: Once it's done, the device returns to the home screen. You will get a usable Cydia app icon.
Users should note that each time they reboot the device, steps 6 and 7 need to be performed for a tethered boot.
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