I have some 30mm Conti's CX tires on my Roubaix, and they work ok most everything except deep sand (or even not-so-deep sand... LOL!)
Ride On
Yuri
EDIT: My Mistake... they are Schwalbe CX Comp
Schwalbe CX Comp - 30 mm
They're 30 mm, but measure to 28mm on these wheels - 17mm internal width