From bug 314775 comment 11:
Upgrading to KDE 4.4.4 was a bit tricky. (...) After everything compiled fine, plasma-desktop crashed each time I logged in to KDE until I removed ~/.kde4/share/config/plasma-desktop*.
I think I found the reason meanwhile: I was using media-gfx/wally-2.3.0 as my wallpaper changer. It uses a plugin for KDE4 so ~/.kde4/share/config/plasma-desktop-appletsrc contained an entry
After deleting plasma-desktop-appletsrc and plasma-desktoprc plasma-desktop didn't crash anymore after login.
I then updated stable media-gfx/wally-2.3.0 to ~media-gfx/wally-2.3.2.
Using wally 2.3.2 plasma-desktop doesn't crash anymore.
So probably we should put a blocker !<=media-gfx/wally-2.3.0 somewhere.
wally-2.3.0 is not in the tree anymore.