If you are planning to jailbreak your iPhone or jailbreak your iPod Touch on the latest iOS 4.2 update, you might want to backup your SHSH blobs so that you can restore your iDevice in case something goes wrong or if you want to downgrade your iDevice.
Though a jailbreak for iOS 4.2 is available, it is untethered, so you will have to jailbreak your device when you restart it so backing up your iOS SHSH blobs might be beneficial in the future. Backing up the iOS 4.1 SHSH blobs is similar to the earlier guide for backing up iOS 4 SHSH blobs.
All you need to do is download Tiny Umbrella from here. Once you download the software just follow the instructions and back up your iOS 4.1. Though a jailbreak for iOS 4.2.1 is already available it is always better to stay on the safer side. Tiny Umbrella is available for Windows, Mac OS X and Linux. The update is a one click process so it should be fairly easy.