iOS (and apparently Lion) deemphasize the users' awareness and involvement in the concept of which apps are "open" and which aren't. iOS already has a popup "mu

iOS (and apparently Lion) deemphasize the users' awareness and involvement in the concept of which apps are "open" and which aren't. iOS already has a popup "multitasking bar" that lets you switch right back to the apps you most recently used. When you switch back they either immediately resume running or deserialize their state back from storage - and the user generally can't (and doesn't need to be able to) distinguish between the two.