It’s been said many times before but a quick reminder is probably necessary at this point…
GeoHot and Comax have now both showcased untethered jailbreaks that are NOT low enough down the chain to be ‘pwned4life’, so Apple will likely engineer out the flaws and kill the jailbreak in a future update. This means that all iPod touch 3G and newer iPhone 3GS owners are well advised to back up their current SHSH signatures to ensure they can jailbreak in the future. n.b. these SHSH signatures are both device and firmware specific so each iDevice must have separate sigs saved for each firmware installed.
Newer iDevices need to have their firmwares signed by Apple and when Apple releases a new firmware they stop signing the old one for activation. If you want to be able to stay on your current firmware, where there is always more chance of jailbreaking, you need to be able to spoof iTunes into signing from a third-party server instead of Apple’s.
This third-party server can either by Saurik’s Cydia server or your local computer. If you are jailbroken now and have Cydia installed then you will automatically have current signatures backed up on Saurik’s server – the front page on Cydia should tell you so (as shown). If you are not jailbroken (or are jailbroken and want to be doubly safe) then you can use a small app called The Umbrella Firmware to back up your SHSH signatures.