The film is called The Hangover, it's being produced by Warner Bros., and is currently scheduled for release June 5, 2009.
According to our helpful contact at the Nevada Film Office, they arrived in town in August for pre-production and only just wrapped, having filmed all over the place, so it was an uncharacteristically long shoot for Las Vegas.
What else can we tell you? Well, it stars Bradley Cooper (Wedding Crashers, "Nip Tuck") and Heather Graham (Swingers, Austin Powers: The Spy Who Shagged Me) and, according to the Internet Movie Database entry for this film, it's "a Las Vegas-set comedy centered around three groomsmen who lose their about-to-be-wed buddy during their drunken misadventures, then must retrace their steps in order to find him."
So, now you know.