This article is from the Ducatis Motorcycles FAQ, by Duke Robillard firstname.lastname@example.org with numerous contributions by others.
Yes, bike rotors can be surface ground with a liquid cooled surface grinder
so long as they aren't thinner than specified in the shop manual after your
done. Surface grinding is more precise than turning. It will not, however
cure warped disks, so if that's the case, don't screw around with your life
and go get the new rotors. Note that if your rotors are vented, the vent
holes must be re-chamfered after grinding. If this isn't done properly, you
will have little hard spots around every hole. You'll feel this soon