The fifth Guild Wars 2 profession was unveiled (at least in part) a bit early today in a "Top 10 of 2011" article published over at PCGamer. The reveal was later confirmed in a Tweet by PCGamer's Josh Augustine.
The fifth profession reveal looks to be the "Guardian", which is said to be the class of Logan Thackeray, one of the game's signature characters. No further details are available at the moment.
Check out the article over at PCGamer.