Mac does not sleep automatically in Lion!
Another safe boot revealed that the auto sleep mode actually is working. I believe that it would probably have worked the last time I've tried it as well  - but, since the sleep occurs several minutes later than the time defined in the Energy Saver settings, I did not notice this.

What about the 'normal' boot? 

I've basically tested the auto sleep with all my menu extras enabled one by one, because the iotop command mentioned by @StuWilson didn't show anything helpful to me. As it turns out the menu extra Stack Exchange Notifier caused insomnia.