[arch-general] Plasma-desktop in KDE 4.7 can not start up

Hi, After upgrade to KDE 4.7, plasma-desktop can not start anymore. Login in from kdm will stop at the splash screen. Switch to console by ctrl + alt + F1 and then run "top" plasma-desktop is using 100% CPU. Run "pkill plasma-desktop" , Login continue. But screen is black. I check .xsession-error file and found nothing special. Does anyone encounter the same problem? How to find more info to track down this problem? Thanks.