If you are someone who jailbreak iPhone or jailbreak iPad or jailbreak iPod Touch then it is advisable to stay away from this update. However, if you want to try out iOS 4.3 you should create a backup of the iOS 4.2.1 SHSH Blobs so that you can restore the device in future or use it for jailbreaking iOS 4.3.
You can backup your SHSH blobs on either Windows or Mac based PCs. To backup the iOS 4.2.1 SHSH blobs, download the appropriate software for your computer from here and follow the instructions to backup your iOS SHSH blobs.
You can use this backup later on to restore your device or Jailbreak your device when one becomes available.