There is some issue related to knotify4, which causes around 100 wakes / second to show up in powertop. It looks like there have already been issues with KDE 4.0.x and there has been improvements since. But it's still too much for notebook use.
The suggested fix is to turn off the notifications, which did not work for me, though. What worked for me was a good old killall knotify4. ;)