Thanks for the tips, folks.
Decided to go with my gut and soaked one of the stones in diesel fuel, then started scrubbing it with a wire brush. Seems to be working pretty well so the soaking suggestions were greatly appreciated.
Dan - I doubt I'd get away with putting them in the dishwasher!
Tom - one of the stones is pretty flat, the other is pretty dished. I've got one of those concrete rubbing stones, quite course, that I'll eventually try using to flatten out the dished one. I don't use these stones for sharpening things like plane irons, chisels or tools that really need a flat stone. Mostly just for my pocket knives and occasionally the kitchen knives.
Anyway, lesson learned: Keep 'em clean in the first place and I won't have to spend half an afternoon getting 'em back in shape.