There's a pretty amazing Android app that I started playing with last weekend at Miller, while doing a track day on the west track. It's called "Trackmaster", and it comes with an accessory program that lets you use an external Bluetooth GPS device to get better definition that you can with the phone's internal GPS system.
You carry this around the track with you and it records absolutely everything. It can even show you the sideways G-forces you're getting as you corner. You can export the data to Google Earth and get a picture of what you're doing superimposed over a satellite photo.
Here's a fairly leisurely lap showing points of acceleration (green) and braking (red):
You can also get what they call a "drive line" that shows your position on the track:
It's not real smooth, but I think it can be improved by tweaking the GPS unit. It seemed to be working at 1Hz, and should work best at 5Hz. There is a 10Hz setting that may be too fast for the phone to deal with.
So you can get a picture like this of every lap you run. Technology has certainly come a long way from the old lap timers. Of course, the lap timer information is also available through this ap.
Cost about $60 for the external GPS and the software with accessory GPS software was about $15. I just stick the phone and the GPS unit in a little pack and strap it around my waist.
Cool stuff... -bg
... but I wonder if the info could be used against me in court? 
... Anybody come out with a Bogey tracker app yet? 