Chris Mooney says it's huge and part of a trend:
Dean was officially the most powerful hurricane that we’ve seen globally so far in 2007, and was by far the strongest at landfall. It was also the first Category 5 Atlantic hurricane seen in the since the record-setting Hurricane Wilma of October 2005. In fact, Dean set some records of its own. Its pressure was the ninth lowest ever measured in the Atlantic, and the third lowest at landfall. Indeed, there hasn’t been a full Category 5 landfall in our part of the world since 1992’s Hurricane Andrew. Dean was in all respects a terrifying storm, and we can only hope that the damage will somehow be less than expected as it tears across the peninsula and then, after crossing the Bay of Campeche, moves on to a presumed second Mexican landfall. [...]
Well, first of all, Dean now takes its rank among the top ten most intense Atlantic hurricanes. If you look at that list you’ll see that six of the strongest (Wilma, Rita, Katrina, Mitch, Dean, and Ivan) have been in the past ten years. That’s not the kind of statistic that’s easy to overlook. According to these data we are getting stronger storms in the Atlantic basin now than we ever have before.
Is global warming to blame? You'll need to read Chris' book Storm World to get a sense for the science in detail, but there's a rough consensus that a warming planet should lead to more intense storms, albeit a lot of disagreement as to how strong that effect is.