For the standard 10-gallon, cylindrical cooler, what's the upper limit of grain I can reasonably expect to mash?
Assuming that this is for a standard 5-gallon batch, and assuming typical mash thickness, you need 1.25 quarts of water for every pound of grain, so the upper limit would theoretically be 25 lbs. (1.25*25 = 31.25 quarts = 7.81 gallons of strike water). The 7.81 gallons of water is displaced by the weight of the grains, so further compensation is needed for space.
This is where I cheated and used the "Can I Mash It?" calculator on the following page, showing that you need 9.81 gallons of space for 25 lbs. of grain: http://www.rackers.org/calcs.shtml
EDIT: Keep in mind you need to also subtract out the dead volume beneath the false bottom if that is how one's equipment is set up. That will take away from some of the volume available for the grain in a sense making your cylindrical a little smaller than 10 gallons.