So the Royals just signed Chris Dwyer to a huge signing bonus for a 4th round guy.  He is a freshman out of Clemson.

http://www.baseballamerica.com/blog/draft/?p=1486Posted Aug. 13, 2009 2:26 pm by Jim Callis
Filed under: Uncategorized
The Royals and Clemson lefthander Chris Dwyer have agreed to terms on a $1.45 million bonus. That's a record for the fourth round, eclipsing the $1.3 million the Yankees gave Brad Suttle in 2007. It's also well over MLB's $240,300 recommendation for Dwyer's slot (122nd overall).

The best freshman-eligible prospect in draft history, Dwyer is a quality athlete with a 90-94 mph fastball and hammer curveball. He came on strong at the end of the college season and turned in one brilliant outing with the Cape Cod League's Cotuit Kettleers before taking the rest of the summer off. 

courtesy of baseballamerica.com

 My question is that how are some college freshmen and sophomores eligible for the draft, while others are not?  And are all juniors and seniors eligible?

Also with the signing day approaching, use this thread to post any and all signings.