CPU time IS given to programs that specifically request it when brought out of focus. Most programs (browsers, games, etc.) have no need to run in the background, but those that do (such as IM apps and skype) will do so so they can still get messages and updates.3ds multitask? No, it doesn't do it to any degree. When you hit the home button, it suspends the software in the background (symbolically and literally). The game that was suspended does not continue to be processed, as in, it saves the program state, stops, then resumes when it is told to. It *cannot* resume while the system menu is being accessed. Also the system refuses to run more than one app at once, so there is no multitasking ability built into the 3ds
Technically, it is multitasking if it can get information like friends that log on, notifications, etc, all while you're playing a game.
Not if it isn't giving any CPU time to the game while you are doing it. Iphone's do the same thing to give the illusion of multitasking by freezing apps in the background then resuming them later.
The programs that don't request it are the ones that get fully suspended (and then drop non-critical data out of RAM after a specific amount of time depending on the program and platform).