According to the LA Times' "Company Town Blog", Infinity Ward is reaching out for help in the development of Modern Warfare 3, the title that is supposedly in development, even after Vince Zampella and Jason West getting let go from the company.
Both G4TV.com and Joystiq report that Sledgehammer Games and Raven Software are currently going to be the studios backing up Infinity Ward. As far as Joystiq knows, Raven Software will be the ones handling the muliplayer, while Sledgehammer is yet to be confirmed on what they'll be doing in this rumored project
Sledgehammer Games was formed after Glen Schofield left Visceral Games when development of Dead Space had been completed. The studio was also due to develop its own Call of Duty title at the time of being founded, but yet to officially announce their title.
Raven Software's most recent game was Singularity.