I'll be there for Christmas and New Year also. It's very easy to find out what is going on out there at that time of the year because all the local are ever so friendly and accepting, as opposed to the high season.
I suggest to have a good read of the the "Ibiza in the Winter" section. Also whilst out there, go to Santa Gerturdis in the afternoon or early evening, and chat with the locals. Another great place is Guarana in Santa Eulalia, Mao's Room in Ibiza Town, etc.
It's all on this site, there's plenty to do and never a dull moment.
However, don't forget to visit the island during the day, I think you would miss something huge by only enjoying the nights out there.....
Have fun
Celine