Changes to the Official Left 4 Dead Website now indicate that a L4D development blog is incoming, much like the TF2 development blog. The new blog can be found here, although there are no entries as of yet. The
official boxart for L4D has also undergone a minor change - the iconic "Infected Hand" is now a left hand, instead of a right - which is more fitting with the theme of the game. Keep tuned for further updates as they occur.

UPDATE: The first Left 4 Dead Blog post has now gone live. 

Quote: "Since we announced Left 4 Dead in late 2006, the growing online
community has been a great source of feedback and support during the
development process. We're planning on using this blog to talk more
directly with that community - through stories, artwork, and insights
into our design and development process. There's also a Left 4 Dead forum where we're hoping you'll let us know what you think of the blog, the game, or anything else L4D-related.

The game is finally nearing completion, and we're down to the final
tweaks before it goes out the door. This week, we're all playtesting
the Xbox 360 version of the game non-stop. There are two kinds of
playtesting at Valve. There's the kind that happens throughout
development, where we bring in people from outside the company, watch
them tackle the game, and then make changes based on their experience.

Then there's the kind we're doing right now, when we're just about to
ship and well beyond the point of evaluating the design or tightening up the graphics.
This testing is all about making the game crash in some spectacular
way. The tests go on more or less around-the-clock and require everyone
at Valve (plus whatever friends and family we can rope into helping) to
playtest in shifts. This is pretty much the only part of the job where
we can say we're literally getting paid to play games. And it sucks. I
mean, Episode 2 is a great game. Until you have to play it 30 times in
a row as fast as you can and also get the "Collect All the Grubs"
achievement or you're fired. This is the dark secret the government
doesn't want you to know about game design: At some point, playing your
own game becomes torture. One company we know calls its weekly playtest
session "The Hour of Fun" out of pure, ironic meanness. After three
days of testing Half Life 2, Gabe Newell's cousin gouged out his own
eyes, turned his bloody face to the sky and began screaming, "I can
still see Alyx!" So what with the constant game playing and the eye
trauma, it's all pretty horrific.

Usually.

One of
the unintended consequences of making L4D, a game that's built from the
ground up for replayability, is that we're actually having a blast
playtesting it. The final sprint to ship has truly been one completely
un-ironic "Hour of Fun" after another. "Company Loves Own Product!"
probably doesn't sound like a huge newsflash. But, in all seriousness,
it's a rare feeling at this point in the process. We're thrilled to be
experiencing it, and even more thrilled that it won't be long before
you're experiencing it as well."