Road shoes tend to be lighter and stiffer (and hence more efficient) than mountain shoes. The downside is that road shoes are generally made with a one piece fiberglass soles (no lugs), which makes them not only difficult to walk in, but will lead to spousal abuse if you use them on your wooden/vinyl floors. In addition, as sorebutt points out, the screw holes for road and mountain cleats are different.

For an example, look at the undersides of these shoes:
Road shoe: http://www.performancebike.com/shop/...egory_ID=2110#
Mountain shoe: http://www.performancebike.com/shop/...egory_ID=2120#

I use CrankBrothers Candy Cs (classified as mountain bike pedals) on my road bike, so I use a mountain bike shoes.