After a bunch of trial rides, I settled on a Specialized tri-cross Sport (triple). It's billed a cyclocross bike, but I'm seeing a couple around here decked out for touring---and I picked it because I wanted the best of a road bike and a mtn, but didn't want a heavy hybrid.
It has more of a road bike geometry than mtn, with some of the comfort features of a hybrid. Aluminum frame, carbon fork & seat post.

The PR campaign says "POINT THE TRICROSS down anything from a gravel strewn fireroad to a pothole packed bike path or maybe even a chunk of smooth singletrack, and you'll realize that its numbness-killing chassis, predictable geometry and robust wheelset will help you redefine what's possible on two (skinny) tires"---and so far I think it's wonderful.

If you can find one in the right size, give it a whirl. I paid about 25% off retail for a small-ish frame, which makes me think not too many sport riders know about it.