I think it makes sense that it's taken a long time for Apple to implement Personal Hotspot; it's a feature that runs in the background that can suck up a lot of battery and add a lot of charges to your plan. I'm sure they had to think a lot about how to best put this into the interface, making it obvious that it's running, easy to turn on/off, etc.

That said, it's my favorite feature of my Nexus One...