We post all of our "outdoor family adventures" (i.e. wheeling trips, highway trips, hiking trips, combinations of the afore, etc) on our family blog. I post the stats in each adventure's post, then I have a gadget near the top of the blog where I keep a cumulative total. This is what it looks like through mid-August 2010 (yes I am seriously delinquent on posting trips)
As far as actually tracking the miles, I just keep an odometer log for the entire trip. Then I split out the off-road miles based on my GPS tracks, known mileages (from guidebooks or other trail reports or past experience), or trace them in mapping software.