Today I updated my system to KDE 4.2 RC1 (factory repository). After the restart plasma crashed. I contacted the #plasma team on IRC and got a good tip within seconds: delete (or better move) ~/.kde4/share/config/plasma-appletsrc.
After that I had a fresh new shiny open source state of the art (or above) desktop environment. If you want to have a look search http://blip.tv for "kde42" or http://youtube.com for "kde 4.2".
The whole KDE community has done an amazing work. Thank you very much! You ROCK!