Interestingly I just wanted to create a thread with my own problems with N9 and WH-701. I've observed the following strange things:

On my N9 64GB with plenty of software installed plugging the WH-701 in results in the charging LED to light up and the the device rebooting. This can be reproduced consistently.

On a N9 16GB which is more or less as delivered by Nokia pluging in the WH-701 once led to a reboot, once led to a failure of the plugging-in to be recognised (payer continued to play on speaker) but on several other tries seemed to work normally.

The only software installed that makes use of the extra buttons to my knowledge is FMRadioN9. Does this software run anything in the background?

Any ideas what could be the cause?

Edit: And yes, the headset works fine with phones that support it officially. So it's no hardware issue on the headset part.