I use Harry's laptimer grand prix edition. Highly recommended! I'll post a vid of my next track day where I use a GoPro black edition to take the video, mic on the rear bumper, and Harry's laptimer for all the car info. Only way to go if you ask me.
Edit: ChrisB I just read your post. Use your gopro for the video, convert it with camtasia studio or your favorite video editing software, import to itunes, then use HLT to overlay. Takes a little effort but you end up with footage like this guy.
Not my vid, but you get the point.