I went to the matrox page and they are providing a code that u have to enter when registering, it's like a 5 step process, and they ask all kinds of questions, then if u change your mind you'll be charged $50
If you're just registering for a free exhibits pass (and not planning to upgrade later to any of the conferences or seminars), you shouldn't be charged for anything. Here's some links to free passes:
"If you plan to attend NAB 2005, Boris FX would like to offer you a free VIP Exhibits pass. Take advantage of this special offer and register today! Visit www.nabshow.com and click Registration. Scroll down to the Other Registration Types section and click VIP Exhibits Pass. Enter Code: MJ0952. This pass is valid for free admission to the show floor if you register by April 13, 2005."