Matt says CBS is going to release a study on DVR owners and ad retention. It seems that folks with DVR's have a recall rate of 23% for ads.
I think it has to do with the "What?" factor. You see something in fast-forward that doesn't quite make sense, so you rewind to watch it. Personally, I know that seems to trigger me remembering and/or recognizing an ad, versus just enduring commercials in "real-time".