The Rebellion Multiplayer Expansion is a series of multiplayer maps and other goodies. "The Rebellion Multiplayer Expansion opens two new fronts against the Reapers: Firebase Jade's jungle reservoir and Firebase Goddess on Thessia," Kotaku reports. "In-game reinforcement packs now include three new weapons as well as equipment, consumables, and six new classes: Quarian Engineer and Infiltrator, Vorcha Soldier and Sentinel, Phoenix Adept and Vanguard." The listing has since been pulled from Sony's site, so we're unable to confirm the contents of the listing.
Update: BioWare community guy Chris Priestly hit the forums to debunk the rumor – at least, about it launching tomorrow. Priestly does say we'll learn more on multiplayer DLC in the future, but doesn't specifically address Rebellion. "We will have more information on ME3 MP DLC in the future, I hope later this month, but nothing more to be said now." [Thanks, JustinSaneV2]