iPhone 3.1 update disables tethering

jole writes: "The newest iPhone 3.1 update intentionally removed tethering functionality from all phones operating in networks that are not Apple partners. This is not limited to hacked or jailbroken phones, but also includes expensive "officially supported" factory unlocked phones. To make the problem worse Apple has made it impossible to downgrade back to working 3.0 version for iPhone 3GS phones."