I bought a pair of these ("The Spoiler" by Slipstreamz) a couple years ago and had moderately satisfying results with them.

http://www.slipstreamz.com/content.asp?subID=9

Nothing miraculous, but they were OK. The other product they sell ("The Slip") probably cuts a lot more wind noise down, but I'd imagine would be hot (and look a little silly, too).