Prepare to be confused.
Okay, so we've heard Infinity Ward is likely cranking on Call of Duty: Modern Warfare 3. Despite the mass exodus the studio suffered after the Vince Zampella/Jason West fiasco, Activision apparently restaffed IW in record time. Then there was the rumor that Sledgehammer Games, a designer owned by Activision, was working on another Call of Duty installment. It was initially purported to be an action/adventure game but was later emended to hold the more expected first-person shooter label. Finally, there was the rumor that Raven (Wolfenstein, Singularity) was working on something CoD-related. Now, we've got what is supposed to be a clarification from the LA Times.
It seems both Sledgehammer and Raven are assisting Infinity Ward in development of Modern Warfare 3. However, Sledgehammer is indeed working on their own CoD spin-off, which is supposed to sport a sci-fi theme of some kind. Sledgehammer appears to be working with IW on the single-player mode of MW3 while Raven is handling development of the multiplayer aspect. There. Does that clear everything up?