The Left 4 dead 2 demo was previously available via pre-order only, but has now expanded to accommodate Xbox Live gold subscribers. There’s no current announcement on when the demo will be released for silver members. The Left 4 Dead 2 demo is now available to download at the Xbox Live Marketplace.
Valve has also announced pre-order bonuses for Left 4 dead 2. By pre-ordering the game on Steam or at participating retailers, you will receive a special in game hat for Team Fortress 2 along with 10% off the title when it’s released.
I am gonna go out on a limb here and say, NO L4D2 demo for Silver Subscribers, since the game is online, makes sense to me. But I have a PC so I can enjoy the free demo all I want.