Maufacturers make it very difficult to get to mileage data over OBDII to prevent tampering....most manufacturers only store it in the instrument cluster itself (well technically the CAN node representing it)....and it varies from year to year. Do a quick forum search and you should find at least one thread discussing exactly this in depth...the conclusion was that it would have to be figured out one vehicle at a time.
The info display in my dash can be selected to show me odometer (and other stuff). Word on the street is that it uses the class 2 bus (SWC) though (not regular CAN). If we just had a big VIN database with special codes in it.... ;-)