When my mark 2 arrives, I am considering the idea of attaching a GPS to the unit and writing some software to help me keep track of business vs. personal miles driven.

Appearently the Infernal Revenue Service (IRS) will settle for simple "business miles driven today" number written in a day timer as valid evidence of your mileage expenses (currently $0.32/mile I think). Now, if my accountant tells me that a printout will also suffice, then I am all over this project.