This is the official FIRST website defining how to qualify for the Nationals:
http://www.usfirst.org/robotics/res_art9.htm
This is the summary portion the above document:
The following qualification process will be used for teams attending the 2002 FIRST Robotics Competition Championship:
All teams must register to compete in a FIRST Robotics Competition regional event before they can register for the Championship.
Teams automatically qualify to register for the current year in the following ways:
a. All former Chairman's Award winners (1992–2001)
b. Prior two years Chairman's Award finalists (2000 and 2001)
c. Prior year National Championship winners (2001)
d. Teams that participated in the original competition (1992)
e. Current year Regional Event winners (2002)
f. Current year Regional Chairman's Award winners (2002) (new award)
g. Current year Technical Award winners at Regional Events (2002)
(Driving Tomorrow's Technology, Quality, Creativity, and Leadership in Controls)
Teams may qualify to register through performance during the prior year (2001) based a point system. Five (5) points are required to register.
a. Regional or National Divisional winner—3 points
b. Regional or National Divisional finalist—2 points
c. Judged Award winner at Regional or National Event—2 points
d. Top Seed at Regional or National Event—1 point
e. Top Rookie Seed at Regional or National Event—1 point