It fills fast and seems to do so before any info ever hits their website, which is typically, if I recall, around December of a given year.
My suggestion would be to write directly to the baseball office at Stanford right after this year's camp and express your interest for the '06 camp. My son received an app and was signed up long before the camp ever appeared on their website.
While others have said it ain't so, my observation was that there is a bit of an "invitation" aspect to this camp. After his inquiry, my son's high school coach and another reference were contacted about him, his ability, etc. He had expressed interest in Stanford, not only just the camp so
I observed a few days of the camp my son attended and there were about 300 kids, everyone I saw was "legit". If this is just some open to everyone sign up deal then by pure coincidence they got 300 real players
I'd also suggest going after your junior year in HS. I recall there being only a handful at best of any kids that were younger.
Like you noted, its a great experience. My son got several offers from schools in attendance there and ultimately accepted one of those. At a minimum, you'll get a real read on where you belong in the college world of baseball.