Just found this thing on another forum and it seems to be really popular. Freakin awesome stuff. Its called Harry's Lap Timer and you can download it on to your iPhone for about $18 for the pro version. Lets your iPhone record HD video and records and overlay track position, speed, lap times, etc. It has most of the big tracks in the US preprogrammed with automatic start/stop built in. I took it for a test run in my daily driver and it really works!
The biggest draw back is that it uses the iPhone's data connection for GPS which can be spotty and only a 1GHz GPS. BUT... if it's not "good enough" for you, you can add on a 5GHz GPS module that just pops on the bottom of the iPhone for $89.
I just went to youtube and did a search for "harrys lap timer" and a million vids come up, this one was with an iPhone...
Harry in Traffic (LapTimer+Dashboard Overlay) - YouTube
it works just like it is supposed to