I put about 750 road miles on my Trek XO-1 with road tires on it, and did not have a problem. I even used it to race in triathlons and had comparible if not better bike times than those people with comparible overall athletic abilities ( ie overall race times). I just upgraded to an ultegra road bike that is about 3 pounds lighter and I only gained .5 to 1 mph over my average on the cross bike. I upgraded because I wanted better components than the Sora, not because the bike was not working for the road.
A lot depends on the bike. My Trek had about the same geometry as a Trek road bike but had a slightly more relaxed head tube angle (71degrees vice 73) I have noticed that a lot of the other cross bikes are using more of a mountain bike crank with smaller chainrings and that might limit you on the road. My Trek had a regular road bike double crank, so I didn't have problems with that.
If you are building the bike from the frame, you can put on a good triple crank that will give you the on road and offroad gears, so you will not have to worry about gearing. Also, I suggest using Cantilever brakes instead of road caliper brakes even if you will never ride off road. I really miss the cantilever brakes for their stopping power. In the rain, you loose a lot of stopping power on both types of brakes, but with the canti's it is much less.
