Sometimes in multiplayer in Black Ops, I see a lightbulb appear over another player's head. What does that indicate?
It's when the player has paused their game. Obviously, you can't actually pause a multiplayer game, but it's when they are on menus and can't move/shoot.
As far as I can tell this is limited to the use of the start menu (change class, leave game) and associated system menus - for example, the Xbox Dashboard.