Comedy Central has John Stewart for the 2010 election, but maybe not much longer. The network renewed Stewart’s contract for two years, but wasn't able to extend him for Indecision 2012.

Broadcasting & Cable reports:

The length of the extension coincides with the 2010 expiration of David Letterman’s deal at CBS, which will fuel the speculation that the Daily Show host and Comedy franchise player could be in line to take over at CBS if Letterman decides to step away after his current deal is up.

Why use the passive voice? Broadcast & Cable is speculating that CBS will close in on Stewart after Letterman retires.

As for Comedy Central, they always Steve Colbert. Of course, that's assuming that his run for president isn’t a thinly veiled publicity campaign for his new book and he’s still available.