I just ran into this article: Arduino: What adapter?.
It says that the external power supply should be 9-12V. That really surprises me, since my Arduino Uno can also be powered via USB, which is 5V.
Later he talks about a 9V pin. But I can only see a 5V and 3.3V pin on mine. Is he talking about a different version of Arduino?
Judging from the picture, his Arduino looks slightly different from my Arduino Uno R3, but I can't tell which version is on the picture.