I searched and couldn't find a thread about this, but Microsoft has confirmed that Xbox Live Silver folk will be able to play certain games online until the new dash hits this fall.
http://www.shacknews.com/onearticle.x/53886
http://www.shacknews.com/onearticle.x/53886
Microsoft has informed Shacknews that Xbox Live Silver members will only have free multiplayer access to select cross-platform titles--Universe at War, Shadowrun and Lost Planet: Colonies Edition--until the next system update arrives this fall.
The representative concluded that, "When the new Xbox experience launches this fall all multiplayer gaming on Xbox 360 will be for Gold members only."