Go to the SpaceX web site and you're greeted by a banner proclaiming:
SOMETHING BIG IS COMING
Click on the banner and you get this YouTube video:
An eagled-eyed observer noted that the SpaceX launch manifest page lists a "Falcon Heavy Demo Flight" for Vandenberg Air Force Base in 2012. The video briefly shows at the end a "heavy" silhouette with an "FH" abbreviation.
The Falcon Heavy is designed to compete with the Atlas V and Delta IV heavy-lift vehicles. According to their web sites, the mass each can lift to Low Earth Orbit are:
Atlas V Heavy 64,920 lb/29,400 kg
Delta IV Heavy 49,740 lb/22,560 kg
Falcon 9 Heavy 70,548 lb/32,000 kg
Of course, the Atlas and Delta models have flown, while the Falcon has not.
UPDATE April 2, 2011 7:00 AM PDT — As noted by Ferris Valyn, the Atlas V Heavy configuration has yet to fly. Atlas V has, of course, flown in other configurations. More on Atlas V at this Wikipedia page.