The T61 has an Intel ICH8 HD Audio device, which is suported by alsa's snd-hda-intel module. So maybe other notebooks with the same sound device might have the same problem. Googling around for a bit i found out that some T60 users report the same problem. So it was either a driver problem or some misconfiguration from me.
Turns out it was not a driver issue, at least not for my kernel version: 2.6.24. So, here's how i got the microphone to work:
First of all, in skype's configuration dialog for audio devices set all devices to "default". Now, from a console run alsamixer and you should see something like this:
This view doesn't matter for activating the microphone, so press
To improve the recording level you should fiddle around wiith the "Capture" and "Internal Mic Boost" slider.
That should be it, just don't remember to turn the recording device off after you're done.