L4D2 is basically what L4D1 would've been if the game had the full Valve blessing from the beginning. If it was a Valve in-house title from the beginning it probably wouldn't have been annouced until 2008 and then released in 2009. But since Turtle Rock announced L4D1 in November 2006 back when they thought Valve was only the publisher; Valve didn't really have a choice but to release it Holiday 2008 after the one year delay from Holiday 2007. People were already pissed that it was delayed when it looked done in 2007, all hell would've broke loose if it didn't come out in 2008.
From the interviews I've read, it seems like the main reason they're putting out a sequel so soon is because Valve really didn't like the way the story was told in L4D1 (as standalone unconnected campaigns). There really wasn't a way to change that major underlying issue without a delay or even through post-launch DLC.
So instead of making new campaigns to fit a structure that they weren't fond of, they choose to do the story properly by starting anew with new survivors. Since new survivors/story structure means sequel, all the new multiplayer, weapon and gameplay improvements that could've been L4D1 DLC would come along with L4D2. As Chet said in Janaury 2008, Valve had a meeting to decide what the scope of the new L4D project that the team was working on since L4D's release. Since Valve employees get to choose their next projects and the team was eager to fix the problems with L4D1, a sequel just made sense.
I for one think that the game is going to be delayed until 2010. The situation really sucks for people who only bought L4D at launch because of the promise of continued support. Faced the decesion of angering fans by delaying L4D1 an additional year to 2009 or the choice of releasing the game in 2008 knowing that it doesn't live up to their expectations, I think Valve that they were stuck with a really tough decision. In the end, we got a great game in 2008 and we're getting the definitive verison that Valve is happy with a year later.