They call it the "Mother of all Bombs" for a reason. Available to all three Strike packages, and unlocked when you nab 25 kills, the MOAB is Modern Warfare 3's version of Modern Warfare 2's Tactical Nuke.
Videos of the weapon in action are surfacing online - here's one for your coffee break. The MOAB is Modern Warfare 3's highest killstreak reward, evaporating all players on the other team, crippling their equipment and grounding their aircraft. Ouch. Unlike the Tactical Nuke, a MOAB launch won't end the match.
Our Modern Warfare 3 review noted that despite the more forgiving pointstreak balancing, "at the top end you still get to drop explosives on people's heads". In retrospect, Mike probably should have over-clocked the phrasing there.