Last year, one of my buddies decided to go at the last minute. We didn't want to wait in the long ass line to register at the show, so he just grabbed one of the millions of lanyards that they were handing out, put it around his neck, then walked in the door with a crowd. They dont really look at your badge unless you walk in through a side entrance or all by yourself. And since there's soo many people at sema, walking into sema all by yourself is pretty damn hard to do.
Another option is to walk behind a really hot chick. Worked like a charm on day 3.
Also, since the sema show is about 1/3rd as big as it used to be, you could skip a day and still see the whole thing. So if you go on the second or third day, you can ask to get a badge from people who have registered thier wives or non-car-enthusiest friends that just came along for the trip to vegas. I've gone for the past 5 years and every single year, I'm with at least 1 person that only goes for half of the first day. Those are the ones that say, it seemed like a good idea until they realize how much walking is involved.