This is on the SB FAQ website
How do I get tickets to Super Bowl XL?
a. The NFL distributes approximately 1% of the tickets to fans through an annual Super Bowl random drawing. The drawing is not considered a lottery (there is no charge to enter). This is the NFL's only method of distributing tickets to the general public. The deadline for entry has passed; it was June 1, 2005.
More information:
Requests for the random drawing are accepted between February 1st and June 1st of each year, preceding the game. For Super Bowl XL the drawing will be in 2005. The drawing is held in October. There are no complimentary tickets available.
Specifics on ticket breakdown:
The two participating teams each get 17.5% of the allotment
Each of the 29 remaining teams receive 1.2% of the allotment (or 35% of the total)
The NFL uses 25% for corporate sponsors, charity and other league related needs
The Host Team (the Detroit Lions) receive about 5% of the allotment