Looks like Mango still has a few hidden surprises. As you can see to the right, that's an official instantiation of Internet Sharing on a Samsung Focus running Mango.
That's the good news, that and it "works like a charm". The bad news is it reportedly eats a lot of battery life. Our bet is this is hidden on some devices and builds and will presumably be a carrier option.
More info as it comes in...
Update: Justin says this is Mango 7720.68 but that his device has been flashed by someone at Microsoft. Insert thriller movie music here.
Update II: Rafael Rivera notes "Confirm ICS is baked into my Mango phone (e.g. Ics*.dll, HKLM\Comm\InternetSharing). My guess is requires carrier "update" to enable."
Thanks, Justin B., for the pic and info!