whats the volume slider in the multitasking bar for? I have just upgraded to iOS 4.2.1 and I don't have an Apple TV so there's no Airplay button, however I still have a volume slider in the multitasking bar?

What volume does that change, and whats the point of it, I can just use the hardware buttons. (I'd much rather it was a brightness slider like there is on the iPad)
The volume changes the volume of the audio on your device, just like the hardware volume buttons do.