Had to get a second headset and opting for a cheap model, mistakingly got one with analogue in- and output. The microphone didn't work and only after a long time - and taking back the headset to the shop to get it replaced - did I find out newer MacBooks and PowerBooks don't have a built in microphone amplifier :(
So if you don't want to build your own amp, stick to a USB headset for your Macbook.