What kind of offroading might you want to be doing? Are we talking tight rocky and rooted singletracks, bouncing off boulders and doing five foot drops? Or are we talking some moderate unpaved and light trails? If it's the latter and you want a bike that can handle long distance and fast road riding as well, then you may want to consider a cyclocross bike. This is a bike that's primarily set up like a roadbike but has accomodations for wider knobbier (although not as wide and knobby as pure MTB) tyres, use cantilever brakes instead of road calipers and are geared a bit lower than a roadbike. The frame is usually a bit beefier and a tad heavier than a road racing bike though.