Transformers: War for Cybertron is now available in US stores offering an action game for Windows, Xbox 360, and PlayStation 3 developed by High Moon Studios to serve as a prequel to the Transformers story. Here's a bit from Activision's announcement describing the game and its multiplayer support, including co-op:

Transformers: War for Cybertron takes players to the final moments of the grand civil war that will determine the survival of the entire TRANSFORMERS race. Fans will be able to explore the TRANSFORMERS war-ravaged home planet in full 3D environments for the first time, armed with a diverse arsenal of high-tech weaponry and the ability to convert instantly from robot to vehicle form at any time as they engage in heart-pounding battles on land and in the air in this gripping, third-person action shooter. Complete with several multiplayer modes, Transformers: War for Cybertron allows gamers to play through story missions with their friends in drop in/drop out online co-op, and also create their own TRANSFORMERS character for competitive head-to-head multiplayer modes, choosing among four distinct character classes, personalizing their character’s look and selecting from a huge variety of weapons, skills and abilities. Additionally, the game introduces Escalation, a four-player online co-op mode that allows gamers to fight together as their favorite AUTOBOTS or DECEPTICONS against increasing waves of enemies utilizing strong teamwork to survive.