On a somewhat amusing/interesting sort of side note:  I'm sure you've all heard of those microchips that can be put under your fingernail by now.  I read quite some time ago that a few clubs in Europe were using them for their regulars -- it lets the bartenders know when they enter the venue and what their favorite drink is so it can be ready for them by the time they make it to the bar.  Plus, it keeps track of their ongoing tab.  It would probably be pretty easy to add to that info a list of their favorite tunage for the DJ.  Personally, though, I doubt the technology is actually going to catch on, but it's somewhat amusing/interesting to think about.