Sounds great, L4D is one of my all time favourite games.I really didn't expect this so soon, after all Valve have only just finished the first game. Did they say which year the November 17th would feature in? We are talking about Valve after all.
There is a bit more info over here -
E3 2009: Left 4 Dead 2 Announced! 4 New Survivors, 5 New Campaigns | Maximum PC