Join Date: Jan 2008
Location: Beaverton, OR
Bikes: 2013 Kona Jake, 2015 Kona Jake the Snake, 2008 Kona Major Jake, 2013 Kona Jake the Snake, 2006 Kona Kula, 2012 Ridley Excalibur, 2008 Surly Long Haul Trucker, 2001 LeMond Buenos Aires, 1984 Pinarello Gran Turismo, 1982 Trek 614, 1979 Austro-Daimler
On the commuter forum people typically claim that knobby tires vs. slicks make about a 2 mph difference. That's only taking rolling resistance into account, I think. Once you start riding with roadies and get up to faster speeds, the wider tires and knobs also start having an aerodynamic effect. You also wouldn't want to try to corner on pavement with a knobby tire at the speeds that roadies will corner on a slick tire.
If you are only going to have one set of wheels and tires for everything, you are going to have to compromise somewhere. For pavement, I'd recommend something like a 700x28 Conti GP 4 Seasons, but that might act a bit like a pinball on gravel. A file tread, as fietsbob suggests, will give you a nice compromise, but it will still be slower than a slick tire on pavement.