The ADZPCTKO (Annual Day Zero Kick Off) is mainly about the current year hikers only it really isn't. Plenty of past and future thru-hikers show up. There are also people who are just PCT groupies. So yes, anyone can just show up and have fun. The only requirement is you care about hiking or about the PCT. The campsites are all full except for 2010 thru-hikers, but there are no problems showing up for a day (please register at the check in desk near the campground entrance so they know you are there, it won't cost you anything).
For anyone planning on thru-hiking the PCT, I recommend showing up a year or two before you hike as you can learn alot at the various seminars. When you are actually hiking, its either too late to change or you are too distracted from meeting everyone and partying to really take it in. I attended my first kickoff in 2007, 2 years before I thru-hiked in 2009. I'll be there again this year even though I'm not hiking.
What I like about the kickoff vendors, is this is one of the only places you can see ULA's packs first hand and get a proper fitting for them. Gossemer Gear is also always there as is 6 Moon Designs, Henry Hillshire's Tarptent, BearValut and Leki. There are sometimes others. And they are usually always available to answer questions and help with any of their products you may have purchased in the past. As an example, I've seen Henry go through the campground helping people correct how they have their tarptents set up to insure a proper pitch. I just wish MLD would show up.