When I was running firmware 3.0, the above sequence (modify iTunes with Terminal command, and then option+click to install an .ipcc file) was EXACTLY what I did to enable tethering. And that "hack" was the ONLY hack I had ever done to my iPhone. And when I installed firmware 3.1 over that hack, I had a LOT of problems with my iPhone. Problems that an AT&T level 2 tech IMMEDIATELY identified by asking flat out if I had installed a carrier settings hack.
I'm not suggesting that nobody do the above. I'm only saying that I will never manually do an update like that again. The hassle of fixing my iPhone (restore as a new iPhone) was just too big!
Mark