I used to be able to quit the ipod app from playing any music by holding the home button on the iPhone 3G for 8 sec but since I updated to the new OS 3.0, this function has stopped working. This problem is caused by the new set of features that comes with iPhone OS 3.0 more specifically the voice features which are activated when holding down the home button. However there is a solution to quit any apps on the iPhone including the iPod and it works if you have upgraded to the new OS 3.0. You will need to hold down the top button on the iPhone (the one used for switching it off) until you see the red slider which says “slide to power off” and then you will need to hold down the home button for 8 sec to quit whichever app is still running. If you were on the iPod, it will close down and you will return to the home screen.
A nuissance I think but fortunately I found the solution.