Mobile devices are designed primarily as a modal experience. You use one app at a time, but can switch between them quickly. Multi-window interfaces and floating apps have been implemented a few times as an alternative, but most of these solutions are a bit clunky. Ixonos has released a new video demo of its multi-window technology, and it looks much better. If only we knew where to get it.
Windows can be resized dynamically by dragging the corner, just like a desktop.