Here’s a prediction for you. The iPad will eventually be able to run multiple applications at the same time. Also, it will run them in a tiling window manager.
Does anything else need to be said? Is this not obvious? A tiling window manager would be the ideal way for apps on the iPad to be run concurrently. Turn the iPad sideways, open two apps, have the iPad tell them they’re oriented vertically, and you’re done! This would be the conservative way to do things, that doesn’t let users muck things up by having overlapping windows. So that’s the way I’d expect Apple to go with this.
Here’s a less hopeful prediction: The iPad will eventually let you run two apps side by side, but not in any other configuration.
But some day, the iPad or some other kind of tablet will have a tiling window manager. You just watch.