Sentry, over on The Verge forums, has some good ideas about how the iOS app switcher/multitasking tray could be improved now the iPhone has a taller display, with more pixels to use.
Some of the mockups suggest showing full screenshots of apps most recently used in the tray, rather than just their icons. This would give a much clearer visual reminder of your position in an app, showing where you were before you switched away.
As iOS devices become more powerful and iOS apps become capable of more, this will become an increasingly useful feature: it can be easy to forget where in an app's interface you were before switching away to reply to an email. Having screenshots taken as you switch away solves this problem. It also makes remembering an app's icon or name less necessary.
Quick access to commonly used settings, such as Bluetooth, Wi-Fi and 3G data seems like a sensible addition to the brightness slider, however added complexity isn't something Apple would be keen to add to their UI. I also don't see how Sentry plans for AirPlay to be integrated in the multitasking tray, as it is now.
I think these UI mockups show what could be made of the app switcher now there's even more room and powerful hardware to play with. I'm sure a future version of iOS will bring improvements here.