http://www.usfirst.org/robotics/2006/cmp_elig.htm
A lot more than you think.
Some are founding teams from the beginning of FIRST's history.
Some are winners of award over the years *chairman's*
Some are merit based on this year. (regional wins, and various other 2006 yet to be determined awards at events)
Then some spots are left open for teams to sign up after a certain time frame this year. (basically a tier system.)