How about in the car? At work? Traveling? I can think of all kinds of ways.
Here's a good example:
A week before 3.0 was out, I was helping a friend load 3.0 on his iPhone at work. I brought my laptop and after 3.0 was done, the phone rebooted and tried to activate. I completly forgot that it needed an internet connection to activate. I quickly went home and grabbed a second iPod cable and connected my iPhone and activated tethering. The guys iPhone activated instantly after that.