This leads to weird contortions in user behavior. For example, my father-in-law's favorite application on his Droid is 'Advanced Task Killer'. From my perspective, the fact that the application even exists is horrifying. A phone should not require sysadmin tools.
In stark contrast, look at how WebOS handles multitasking. You know exactly what's running at any given point, because you have a visible card up. To close an application, flick the card up and off the screen. It's intuitive and obvious, even to non-technical people. It's really an amazing design.
My suspicion is that Apple will never allow multitasking on the iPhone unless they can make it Palm-like in it's simplicity. However, that would probably require a major UI refactor, which is not going to happen any time soon.
EDIT: This contains some bad info. See thread below.