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!]