I've had wifi tether on my rooted G1 for quite some time now. It's not something I use all the time, but it's a huge convenience when you're out of the office or house. Where in the past I might go looking for an open AP to hop online real quick to take care of something, now I just tether my laptop to my phone without worrying about it.

A second battery would be pretty great to have on a long flight when you'd actually like to make phone calls after using your phone as a music player/gaming system during the flight. :-(

I know two iPhone owners who carry two phones for just that reason. An iPhone for for all the app and internet goodness and an old phone, which can easily go at least a week without needing to be recharged, for when they actually have to make calls.