Kotaku has posted a podcast with Chet Faliszek, taped a couple weeks ago, where Chet said Valve would be announcing their plans for downloadable content (DLC) shortly.

He mentions it twice in the 21 minute podcast, the first is near the beginning and he says:

So one of the things we're going to be announcing really shortly is some DLC on the PC and the 360. We'll be having both the Versus maps and some new stuff coming out.

And the next time is near the end:

If it wasn't the holidays and people weren't taking all these vacations here at Valve, you'd have seen that announcement already. We should be announcing that before Christmas - what the DLC is. The holidays aren't so much delaying it.. the press guys.. Doug has been taking some time. We'll have an announcement shortly, I don't know exactly when. We've got the DLC coming. We're going to do the 360 at the same time as the PC, and then we also have on the PC side the SDK coming out.

It looks like we'll have Dead Air and Death Toll on Versus mode and maybe some new maps. The rest of the podcast talks mainly about the story of Left 4 Dead and the way the story is told and how much Chet likes working at Valve. Go listen.

http://kotaku.com/5118863/chet-faliszek-talks-left-4-deads-future-old-man-murrays-past-and-how-to-write-subtly