irvined,
I've been looking into doing something exactly like this for a few months, but haven't been able to figure out what kind of sampling rate is possible while writing to an SD card. Do you know how fast this is capable of polling sensors, getting GPS info and writing to SD? I'm also working on data logging for racing purposes, and wanted to try rolling my own DAQ system using Arduino, but am not as experienced as you seem to be, so it is taking a lot of searching... then I found this page. Awesome blog you've got!
Thanks
Justin