I agree with Andy_K on the Michelins, they roll much faster than knobbies (though they tend to pick up some gravel in the treads.)

If you're willing to go with a tire as slick as those Kenda 838's, you might want to take a look at the Kenda Kwest also. I've had a few sets of them, and they're great on the road an on hard packed trails also. I've done plenty of non-technical off-roading with the 26x1.5" 100psi variety on my Rockhopper: