According to getbig.com:
Dexter Jackson - Dexter Jackson was so mad and pissed off and hurt that he got booted out of 3rd place because of the 'inaugural' challenge round, he opted to compete in the Arnold Classic and take first place, beating out Chris Cormier by 1 point. As for this years Mr. Olympia, Dexter is nowhere to be found. Why should he. He is the king of the Arnold Classic, where his physique is great. He doesn't need to be at the O. Besides, he has a big announcement to make at the Olympia from the Met-Rx booth. We are all wondering what it is?
Also, a good read as to why Lee Priest pulled out:
http://www.getbig.com/news/2005-04/100605lee.htm