Cerberus Network page, part of the official Mass Effect site. BioWare co-founder Greg Zeschuk described the Hammerhead as a "hover tank," during a recent interview with Joystiq, adding, "It's not a wheeled vehicle, so it's going to have a bit of a different control scheme" than did the much-maligned Mako vehicle featured constantly in Mass Effect 1.
On the website, the Hammerhead is promoted as "a heavy assault vehicle that hovers over the battlefield at up to 120 kilometers per hour [75 mph] and features a guided missile system ensuring accuracy even during aggressive maneuvering." In other words, it's everything the Mako wasn't -- and it's optional.
The Firewalker DLC pack is scheduled for a "late March" release and will be accessible free of charge through the Cerberus Network in-game delivery system. Of course, nothing's really "free" when it's got Cerberus' shady name attached to it, is it, Mr. Illusive Man?
[Thanks Runyon C!]