[lunarhighway]

i think that would depend on where the odometer gets it's signal, if it's on the driven wheels it would record the extra distance, if it's on the non driven wheels it wouldn't, also turns might affect the actual distance vs the one recorded depending on what the odometer actually reads. different tire diameters will also affect the accuracy.
Some gps devices can record the distance traveled as wel as show the speed, and this will generally be more accurate as there are no mechanical variables.

I do think however you'd have to spin your wheels more than the occasional slip for it to have a signifficant effect on your milage figures, especially since the propper response to spinning wheens is to lift of the throttle and let them slow down so they can regain traction.