From reading about the program, not everybody gets selected.  How many people they select for each room depends on how many classes they are teaching and how many people are attending the event.  And as tturner mentioned, STI students get priority.  2nd page of this document (http://www.sans.org/security-training/volguidelines.pdf) shows the priority listing: STI students, people who have facilitated previously, then first timers.

I've applied twice.  I didn't get chosen for the first event (one of the big SANS events that was half way across the country), and I'm still waiting to hear back on my 2nd application (community SANS event right here in town).  We'll see what happens!