If I were CBS, I'd still want Stewart out of the two Comedy Central guys. I feel like he'd have more appeal to a wider demographic, and is a more known quantity as far his interviewing skills go(since Colbert the character as an interviewer might not have much to with Colbert himself as an interviewer). But maybe Stewart's not interested?
Here's a wildcard: how about Conan? He found a home on TBS, but it's still basic cable. His current contract runs through November 2015, so a little later than you'd like if you're CBS looking for a replacement. But if they came knocking with an 11:30 network time slot, does Conan say no? I think it'd be tempting to him, but also knowing his personality, he probably feels a degree of gratitude/loyalty to TBS for giving him that opportunity when really no one else had time/room for him on their schedules after he left NBC, and for giving him basically free reign there. Actually, now that I think about it, given that Conan owns his show and the 12:30 show(it's Pete Holmes now, right?), chances of him leaving are probably slim. And would CBS even be interested in a former Tonight Show host, particularly with all of the events that occured in 2010?