Get one with an inverted pattern. There will still have some knobbies on the side of the tire, but flatish in the middle for the road.

I think some people call this semi-slick. Maybe inverted is the wrong term, but anyhoo: http://bicyclewarehouse.com/itemdetails.cfm?id=8598

This is what you should get. Well, not exactly this tire, but this style. I never tried this tire myself. Looks like a good tire so try it and see for yourself. I'm sure someone else will chime in with a recommendation.