In an e-mail response to a fan, Valve's Chet Faliszek made a statement that confirms a patch will arrive "this week" for Left 4 Dead.
Sadly Microsoft has nothing to do with the pc patch... that delay is all us. The PC and 360 will be patched independently not sure where people picked up that exploit/bug fixing was tied together. Only the DLC will be simultaneous. The problem on the PC side was a few large bugs affected other systems that we were going to rewrite on the pc so we ended up waiting to fix a whole class of bugs instead of just individual bugs. The patch should be out this week.From what we can gather from this mail this means that only fixes are coming this week. The DLC may very well be pushed back to a point in the near future.
Thanks
Chet