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Same here

i just upgraded to 3.1.2 and still had the tethering option but i connected it to the USB cable it went away - sigh. at least mywi works great.

I had the exact same scenario. I thought i was still good, until i tried to actually use it... then poof.
 
This actually happened my friend who upgraded straight to 3.1.2 to get MMS but his tethering stayed on. I checked his firmware and he was not jailbroken. Also in his general settings it said profile and he had the Ben.M profile installed. Everything was working to MMS, VVM, and tethering all on non jailbroken 3.1.2.
 
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