I'm a u-scan op/checker at Kroger, and I can confirm the filth of the belts. Ours are cleaned nightly (and daily too, if we're not busy) but they are almost instantly dirty again, so it's almost pointless. In addition to the wide variety of contaminants already there, a surprising amount of people seem to think it's ok to let their children walk around on the countertops.
Can't complain too much though. I would never have tried to quit eating meat if I hadn't been forced to handle big, leaky packages of it all day.