Since precisely this morning I have the problem (and yes, I did several reboots) that the side-buttons of my N9 don't work as usual, but the power key triggers (when used to lock the screen, NOT when used to wake up device instead of double-tap) the volume-up-key once.
The volume-up-key on the other hand works with the function of the power-button, meaning it locks the device.

What the hell went wrong there and has anybody encountered that before?

never encountered such.
have u tried soft reset.
if not soft, hard reset?

lol, Phone PMS? :P Yeah, try soft reset or reflash.

I don't really have the time to flash around at the moment, not sure whether its software-related..

I still have warranty, and since I'm not sure whether its soft- or hardwarerelated I will probably try to get a replacement device from the dealer or Nokia..

edit:
actually i will try to reinstall from the software-suite once, before taking it to our local Nokia-Care-point