Perhaps you already read this Sheldon Brown article on mechanical advanage; but here it is just in case. Once again, Sheldon Brown to the rescue:
http://sheldonbrown.com/cantilever-geometry.html
Toward the bottom is a section on V-Brakes and STI shifters.
Contrary to advice from above, you can't just space the brake pads in toward the rim. It will look like the brakes are set properly; but there are leverage issues involved, not just how close the pads are in relation to the rim.
Lastly, most cross bikes do not use V-Brakes because cantilevers work with STI shifters without adapters.