According to this RPS interview with Valve's Chet Faliszek, the team is working on character-driven promo movies, similar to the ads release for TF2.
"We’ll do something like that for Left 4 Dead," said Faliszek. "We have a bunch of guys working on it. We’re hooked on it. They’re too funny. Speaking as someone who doesn’t actually have to do that work, I’d say that they’re totally hilarious. Yeah, I’m sure the team don’t think so. It’s a long slog to get the funny."
The TF2 shorts were very cool clips showing each character class and giving a taste of Valve's take on their personalities. This, then, is good news.
There's no release date for Left 4 Dead at the moment, but expect to see it on PC and 360, at least, in September this year.