the survival pack for left 4 dead released today on xbox 360 (not sure about pc) and it is awesome.
the new survival mode is ridiculously hard with the difficulty ramping up exponentially. there are 5 maps to play on so far, 4 of them coming from the existing campaigns and 1 of them being new. my best time so far in 3 games is only just under 4 minutes but i havent yet been able to build a team of friends to get it done right.
they've also added the 2 campaigns to versus mode that had been left out, and tweaked the gameplay a bit to make it more balanced.
when i picked up left 4 dead brand new at launch for $60 i was a bit disappointed with the amount of content even though what was there was awesome, but this free update completes this game making it fully worth the price.
edit: a little update on the status of the dlc just before launch had valve saying they havent yet been able to get past the 9 minute mark...