Earlier this week, Apple released the GM seed of iOS 4.2, which among other features, brought a volume and brightness slider to the iPad. At the time, it was believed the sliders were iPad only.
After a little searching, it seems that the iPhone/iPod touch on iOS 4.2 had the volume slider the whole time! It was hidden!
The volume slider is accessible in the multitasking dock all the way to the left. The first slide to the left takes you to the iPod controls and orientation lock, but if you slide once more, the volume slider is revealed.
Furthermore, AirPlay is now easily accessible on iPhone/iPod touch next to the volume slider on the multitasking dock!
Thanks to @neodoru for the heads-up!