Left 4 Dead will be one of the officially supported games at the next LAN in April.
More details will follow but in the interim grab it cheaply while you can from Steam for £13.49 (*THIS HALF PRICE OFFER COULD EXPIRE IMMINENTLY - MOVE QUICK!!!)
Details here:
http://www.deadmen.co.uk/forum/showthread.php?t=26364
alternatively it's cheap here:
http://deadmen.co.uk/forum/showthread.php?t=26176