Hi, I'd like to upgrade my OS from 3.1.2 to 4.x. Can I do this, yet, given I have jailbroken my iPhone 3GS's 3.1.2 OS? I only use jailbreaking for tethering, so it is imperative that I can continue to tether when I upgrade. I am on AT&T. Is there a method available for upgrading in my case? Thanks!