Sounds like you want a dual purpose tire, like the Continental Town and Country or the Nashbar Elevator. They have a nearly unbroken center strip that gives a smooth ride on pavement, but they still have some knobs on the side for dirt. They're either the best of both worlds or the worst of both worlds, depending on the type of riding you do most.
We're going to be doing a lot of asphalt riding as compared to mountain biking, so this tire may be a good choice. Is tire still useable once it's been removed from a rim, or does this break the "seal" and deem the tire useless? I figure it'll be nice to have extra sets of tires when we do hit the trails...