It took me ages to figure this out. Seems everyone and their dog knew about it. For years, I’ve been deftly flicking through the Nokia menu structure trying to get back to applications running in the background.
Once, my pure mistake, I kept my thumb on the menu key for a second longer than necessary and this little ‘alt tab’ window popped up.
A nigh-on revelation for me when I’m flicking around things on the E61 and the N93.
I really should RTFM.