I hooked up my Garmin Edge 500 GPS cycling computer to the forks of my 450 and did some motos the other day. I used the "Lap by Position" mode, then "Start and Lap" mode. Went out on the track and pressed start where I wanted to record the lap time, and put in a moto. The track was lined with trees, so I think the data was a little sporatic on some laps, and I need to set the "Start" more to the center of the track. I set the "start" position off to one side of a table top, but my riding line was on the other side of the track.
You can see the data here.
http://connect.garmin.com/activity/213877787The company I work for is designing a billet mount to get the unit off the fork tube, and just behind the bars. I love the Garmin for the bicycle, and now I can use it for the dirtbike!