Computer scientists at the University of California, Los Angeles developed an interesting solution for social DJing dubbed Smart Party.

Smart Party is polling via Wi-fi the playlists of the music players party guests would carry. This would work for instance with the iPod touch or the Microsoft Zune players. Each collected playlist is then sent to a PC where a software analyses the most popular tracks and genres.
Smart Party would then just play the most popular songs amongst all party goers which makes those self-declared party DJs a thing of the past.
One of the technical issues is DRM, but with the move (hopefully unstoppable) to DRM free music that should solve itself soon.
More details on the NewScientist.