Even though pod2g delayed the release of A5 jailbreak, we know that we’re just a few days away. Both redsn0w and Corona untether (or Greenpois0n) will sure be able to jailbreak your iPhone 4S and iPad 2 on iOS 5.0.1. To protect yourself in the event of an “accidental upgrade” to a newer firmware iOS 5.1, which would make you lose your jailbreak, you will have to save your SHSH blobs.
What is SHSH and why save them?
SHSH blob or ECID SHSH is actually a signature check which is verified against Apple servers whenever you try restoring the firmware on iPhone, iPad and iPod touch. As soon as a new firmware is released, Apple stops signing the older firmware, making it impossible to restore back to the older firmware.
In the event of A5 jailbreak release, Apple could rush to push iOS 5.1 update to patch the exploit, but if you have saved your SHSH signed blobs for iOS 5.0.1, you can then restore your iPhone 4S or iPad 2 to jailbreak-able iOS 5.0.1. These backed up SHSH blobs will help you to downgrade your iDevice firmware to previous firmware as Apple usually stops signing them after a new iOS software update is released.
How to save SHSH Blobs?
You can use TinyUmbrella for saving SHSH blobs of your iDevice. Recently, developer notcom updated TinyUmbrella to version 5.10.05 fixing a nasty crashing bug in Windows version. This fix affects you if TinyUmbrella would crash on startup with “LibraryFinder” errors in the log including “unable to validate libraries” and “Reticulating splines” errors. Hit up TinyUmbrella and download the latest version TinyUmbrella 5.10.05 for both Windows and Mac and follow on-screen instructions to save SHSH blobs.
The hacker pod2g strongly recommends iPhone 4S and iPad 2 owners to update their device to iOS 5.0.1 firmware (not iOS 5.0) to enjoy untethered jailbreak. Have patience till pod2g finds an alternative way to find a distributable exploit that could inject the untethering files to the iPhone 4S to achieve stable iPhone 4S, iPad 2 jailbreak.
[UPDATE]: As noted by Veeence, if you are on an iPad 2 3G on iOS 4.3.3 and want to upgrade to iOS 5.x, you should upgrade to 5.0.1 ASAP (and wait a few days for the A5 jailbreak). If Apple pushes 5.1 update, they will stop signing 5.0.1 and chances are you won’t be able to update to 5.0.1 (even with your SHSH blobs for 5.0.1). And that rule actually applies to iPad 2 WiFi as well. SHSH blobs are pretty worthless for those devices (A5) ATM.
[UPDATE 2]: Per Musclenerd, iPhone 4S unlockers MUST not update to 5.0.1. Here is the full note issued by the hacker.