It might stretch the boundaries of the celebration's No Commercialism policy, but in-world communities such as Virtual Ability, Luskwood, Relay for Life, and others could stand to benefit greatly from focused exposure at such a large event.
[Update 17 April 2012 12:19PM]
STOP! Go read "Building Community in Second Life or Not" by Honour McMilan NOW. It seems there will be no SL9B at all in the traditional sense. The Lab wants us to celebrate their birthday without them.