So iH8sn0w has released the newest version of his iTunes Error By-Passer (iREB).This is the third version and has some new updates. Including an nicer interface, credits, and the ever important SHSH grabber for the 3G[S] that will help with the ability to continue to Jail Break and downgrade no matter what Apple has in-store.
It is also much more stable for those of you that were having troubles.You can download it here. Check out some screenshots below:
FSM EXCLUSIVE DEAL Later edit:
The main purpose of iREB is to fix iTunes error 16xx and 21. SHSH Grabber, purpose is to ensures the ability of a future downgrade The operation is simple and is clearly shown in the video below. To find out your 16 digits ECID code, you can follow this guide.