Just stumbled across this app for the iPhone:
Dynolicious, an automotive performance meter (http://www.dynolicious.com/).
Apparently, this iPhone application uses the accelerometer to provide performance metrics while you drive. According to the web site, you get metrics for:
1/4 Mile Elapsed Time
Lateral G's (huh? G-force stuff? cool!).
Not that I'm any kind of a performance driver, but my little Civic SI Coupe has a little bit of power to it and it would be fun to see what the iPhone says about it.