I just mounted a rack on my road bike. The bike does not have rack attachment points on the dropouts, but my LBS dug up a set of Blackburn "custom eyelets" for "Campagnolo style dropouts." These are threaded metal inserts that fit inside the small triangular opening in the dropout. The bottom of the rack gets attached with screws to the inserts. The folks at my LBS [Bicycle South, in Decatur, Georgia--they are great!] recommended strongly against using clamps for the bottom attachment because the seat stays are tapered towards the bottom, and they said the clamps would slip down with any significant load.
For the top of the rack, I used the clamps that were provided with the rack to attach the brackets to the seat stays.
The rack seems very sturdy, and I'm glad that I was able to take a bike that was designed primarily for racing and make it more useful for commuting. Now I get to go decide how much money to spend on panniers.