There are really two different uses here:

1. Being able to quickly swap between different applications.
2. Having two applications that actively multitask.

For number 1 above it is enough to create a persistent copy of the application being defocused. As far as I understand (which is very little) of Android, this is the path it takes. I have to admit that most of my "multi-tasking" fall into this path. Examples of this is keeping ebook reader open at the same time as the browser or an xterm with ipython, etc...

For number 2 you have multiple applications being scheduled by the scheduler and they both need their share of the CPU, and they really "run at the same time". Examples of this that I wouldn't want to live without are the fact that you can run Conboy and the the calendar at the same time your are in the middle of a phone conversation.

Other examples for number 2 are various monitoring and notification applications.

Something that I do miss is the ability to have more than one application visible at the same time. I find that most of the writing I do need some kind of reference. This goes for programming or an email while referencing material on the web etc. The resolution of the screen is high enough to keep two narrow windows open in landscape mode, but the maemo window manager does not support it. Let's hope that MeeGo fixes this.