I am using my iPhone for a few years now, I bought one about a month after they were released first - yeah, that is how long I could refrain myself from being the typical fanboy. While I use quite a list of apps more or less daily (I will post about this later) I found that I still do not know all the little tricks it has in store.
Only recently I wondered why I sometimes see the iPod controls on the locked start screen. After fiddling around for a few minutes I found that double clicking the button reveals these controls. First here is how the normal locked screen looks like:
Now below you can see how the screen changes when you double click on the main button located below the screen. It allows you to use the iPhone more like an iPod without the need to unlock the device, click through screens to get to the iPod application - something even worse when you have already clicked away before the device was locked, for example listening to music while reading emails.
Once you have the controls you can start and stop the music and navigate back and forth through your playlist. I used to do that with the button in the microphone of the iPhone headset. Which is still fine, but this is an alternative in case you have your phone already in your hand. Here the screen shot of the iPod controls:
By the way, double clicking a second time hides the controls again.
Another little trick is if you have successfully scrolled through the pages of applications and web links on your iPhones main screen and you reached page 5 or so you suddenly realize that you need to look up an address using the Map application. Sure you can "swish and flick" your way back to the first screen, but a much easier way is to single click the main iPhone button positioned under the screen again. It scrolls all the way back to the first screen. Nice!