I was trying the wrong headset, the one supplied with the Nokia 5800. I found a headset in the jewel box of my girlfriend with an extra contact in the 3.5mm plug, and I found a small hole on top of the little box which should presumably be a mike and a break button. See photo. A few years ago Nokia supplied this kind of headsets with their phones.
I tried it out and it worked on both the N810 and 5800. I am still unsure whether this was the headset supplied with the N810.