I'm not sure how long this has been around; I'm expecting since 1.1.2 released. If your iPhone locks (sleeps) when it's inside of any application, you can quickly return to the home screen without extra button pushes:

1. Lock your phone while in an app, like the iPod.
2. Press the Home button to wake it.
3. Immediately swipe to unlock. Speed is key; usually the swipe has to be started before the screen lights up.
4. When the iPhone unlocks, the iPhone returns to Home screen instead of the iPod. (Some apps seem to be tougher to do this with than others.)

This doesn't happen when the phone is awakened with the Lock button, and it's clear that the application isn't crashing, because you can occasionally grab glimpses of the retreating application's animation and the Home screen's animation.

A quick poll of my two iPhone-owning coworkers think it's a bug, but it's a slightly useful one.