Ever hear of the Midnight Riders? Think about the "Dark Carnival" campaign in Left 4 Dead 2 -- ring any bells?
For those who still haven't got a clue, the Midnight Riders are a fictitious rock group in Left 4 Dead 2, and players see posters for their concert all over the fairgrounds. In fact, if you hang around one of the safe houses long enough, Ellis (or "Ellie," as I call him) and Coach begin chatting about the group, with more references throughout the corresponding mission. Apparently the band was known for their kickass stage effects, including lasers and fireworks.
Seems Valve is planning to do something with the group too. A Midnight Riders website was discovered in these past couple days, though there's nothing on the page except artwork of the band members and a "Coming Soon" notice. Glancing at the page info, we see the site was last edited on November 17; seeing as how it was picked up by most media outlets today, it may have launched just yesterday.
There's nothing solid on the Midnight Riders project, so we've only got the teaser website to go on for now.