The LA Times is also speculating/suggesting that Colbert will wait for fall. Between Memorial Day and Labor Day, CBS might explore options other than Letterman re-runs.
Between Parr and Carson (ABC would not let him out of his contract early), The Tonight Show had a series of interim hosts. The Ed Sullivan Theater will be undergoing some renovation, so CBS could move the show temporarily to Television City or Studio Center and bring in interim hosts...
Jay Leno (still not pleased to be pushed out)
Still under contract to NBC, and probably not all that interested in a stunt gig.
Dick Cavett (probably would appeal to Letterman's crowd)
Is he still alive?
Colbert's runners up (NBC passed on Merv Griffin but so impressed them as interim host, they gave him a daytime show)
I can't see modern TV execs willing to use the Late Show slot for on-air auditions. Besides, if any of them turn out to do better than Colbert does when he starts, that will make the suits look bad, and suits don't like to risk looking bad.
Howard Stern (if he could stay PG13, he'd be great)
Even if he doesn't, the show is "live on tape". With a several hour tape delay, they could clean up anything he did.
Stephanie Miller (very funny)
You're kidding, right?
Al Franken (while Senate's not in session)
Franken wasn't all that funny before he became a political joke. He did write some funny bits, but they required polishing and rehearsing. He never displayed any talent for thinking on his feet. And since entering politics, he hasn't displayed any talent for thinking, period. Aside from being able to draw the United States map from memory, he just doesn't come across as very bright.