As I did mentioned today I did made a mistake. The Centre pin of the power jack is negative. So the power Jack of boards of revision 2.1 or earlier (there is no completed newer revision yet) is not compatible with the power Jack used on the Arduino Uno. The documentation was also wrong on this. I did change the details section and added it to the errata of rev2 and rev2.1. A wrong power supply do not destroy the board because there are reverse polarity protection diodes.