I just stumbled across this. It's an APRS tracker app for the iPhone, called iBCNU. It uses the phone's built-in GPS to send out position reports periodically using the 3G network or wi-fi. Since it isn't using ham radio it is a bit like cheating, but considering the problems I've discovered with lack of ham radio APRS coverage in rural areas it seems like the best solution.
The display is really cool. It even shows your current Maidenhead locator, which would be great for VHF "rover" activity. Even the VX-8 can't do that. Besides position reporting it also supports APRS text messaging.
It's a shame the iPhone is so expensive, and is only available here in the UK from O2 at the moment. Mind you, the VX-8 wasn't cheap. Since I already had a 2m HT, getting one of these would probably have been a better way to get the APRS functionality I wanted.