To qualify for a ticket, you need to work within the industry. That is a pretty vague field. You can do anything from work for a dealership to owning a fabrication shop.
The point of the show is to allow manufactures and buyers to meet and create business deals. The cool thing about it is all the products and cars on display. It's like any other business, flashiness sells, so there is lots for the eyes to see.
I just got back from SEMA 2008 with a lot of information and I saw a lot of cool cars. If you get the chance to make it to SEMA, you should!