I know of several MB dealers that have used http://www.paspeedo.com/ to program mileage following programming errors at the dealer. I've sent a couple there myself.

I'm sure there are other "companies" that can do this as well. I think that any "shady sales lot or thief" could get the mileage rolled back without too much difficulty. The problem is (for the thief), it wouldn't be too hard to detect. A VMI will show the date and mileage of all warranty work, including Recall Campaigns. CARFAX can sometimes be useful as well, but I'd go with a VMI first. On a newer MB, spotting a significant mileage discrepancy would be pretty simple.