It really depends on what you mean by "Watchmen," ultimately; can the core plot of Watchmen (Solving the murder of a superhero and the larger plot it unveils) be made into a movie? Of course. But can the full experience of the Watchmen comic be translated into a movie? Probably not.
The problem is that Watchmen, more than most comics, rejoices in its medium; there are narrative techniques used throughout that could only be achieved in comics, and that was one of the points of the comic when it was being created. For that reason alone, Alan Moore told (onetime possible director) Terry Gilliam that Watchmen was unfilmable.
Much of that may be Moore's distaste for cinema seeping through, however (The writer likened mainstream movies to being forcefed regurgitated worms in a recent interview: "The 'Watchmen' film sounds like more regurgitated worms. I for one am sick of worms. Can't we get something else? Perhaps some takeout? Even Chinese worms would be a nice change"); while the Watchmen movie won't be able to recreate all of the book's trickery and content, co-creator Dave Gibbons has been promoting the movie and outspoken in his support for what director Zack Snyder has done with it. Given that reviews have ranged from the insane fanboy excitement to the comedic fanboy disappointment, whether or not it really is unfilmable or not may be left up to your own judgment.