If you read the actual requirements[1], they're going to be hard for a lot of people to meet. For one, you have to be able to drop everything for about two weeks near the end of the year, then about a third of 2013. You have to have a bachelor's degree, plus 3 years of experience (grad school counts for this), have to pitch a personal research project to them & have experience doing field research, be in good physical shape, not mind being cooped up with five total strangers for four months eating astronaut food, do all kinds of strange tests, etc.

I'm guessing the graduate student exception to professional experience is because that's where they're most likely to find reasonable candidates for this, because they might be able to take a semester off and possibly even get academic credit for doing something like this. Most other people couldn't just leave their jobs for a four month gig with NASA, however cool it might be.

Anyhow, it sounds like one of their goals is to have people who could be trained as NASA mission specialists. So if you want to be an astronaut someday, this might be your chance.

[1] http://manoa.hawaii.edu/hi-seas/