You might say it's slimy, but turning it on its head (and without looking at the new APIs, etc.): Apple realised with Camera+ that there was a need for a physical button for taking a picture, the inclusion in iOS5 of this feature means that all camera apps (presumably) get the ability to use the physical button, rather than the camera apps hacking it in.

Again, I don't know if Apple is opening this up, so I won't describe it as slimy or not.

Someone like Red Pop [1], however, might be worse off though...

Edit: Re-reading the rejection notice from your link [2], it seems clear that Apple won't be able to reject people based on that clause again. At least, using the volume button for camera now seems to be pretty standard (IANAL so YMMV).

[1] http://www.red-pop.com/

[2] »Your application cannot be added to the App Store because it uses iPhone volume buttons in a non-standard way, potentially resulting in user confusion. Changing the behavior of iPhone external hardware buttons is a violation of the iPhone Developer Program License Agreement. Applications must adhere to the iPhone Human Interface Guidelines as outlined in the iPhone Developer Program License Agreement section 3.3.7«