While the rumor is that Valve is currently mid-development on Left 4 Dead 3, a leaked image today reportedly shows the studio is still dabbling in Left 4 Dead 2 content. The image shows a slideshow of shots taken of Left 4 Dead 2's Plantation level updated in Valve's Source 2.0 engine. Needless to say the improved detail is very impressive.
However, let's not make this story out to be any larger than it is. There's no reference to Left 4 Dead 3 or Half-Life 3 and no other implication that this is anything more than a tech demo to show to developers. The slides along the left-hand side show comparison shots of different areas in the plantation level, all very impressive, but otherwise inconclusive with regards to Valve's secret plans to take over the world.
Perhaps Valve is planning on releasing Left 4 Dead 2 updated in Source 2.0? Or perhaps they're working on a map pack DLC update for when they finally release Left 4 Dead 3? Those all require a bit of healthy speculation, but considering how great these screenshots look I don't think anyone would be opposed to the idea. Even if they're just a teaser, I think they're a teaser we can all appreciate. Bring on Source 2.0, Valve.