Out of limited resources, Apple chose to make the UI fluid, at the expense of things like multi-tasking.

Out of limited resources, Nokia chose to make features like multitasking work, at the expense of things like UI fluidness.

Eventually, the hardware will become powerful enough to support both. Then you will see the iPhone multi-tasking and the N900 successors with a very fluid UI.

If you have been following the hardware market, we are very quickly reaching that point.