That's visual multi-tasking for the end user. N900 already does "true" multi-tasking technically. (I can even play an xvid video in the background while browsing the web. NO phone can do this except the N900.)

In saying that, it actually would be really cool to have 2 apps showing simultaneously on a mobile phone or tablet. That would definitely be interesting and very different to anything else out there.

eg. Web browser on one half and a avi/mkv video playing at the same time on the other hlaf. Now this would be AWESOME.

In fact, WHY don't anyone do this? This is a gold mine of an idea imo.