Hehe... Spoke too soon. I was adding a Plasma widget to the Panel, and it froze. Rebooted from VT1 and... Plasma Desktop is segfaulting again. Arggghhhh!!!! KDE.
Back on Fluxbox for the time being.
EDIT: Just re-downloaded sabayon-sources-2.6.38-r1 and am installing nvidia-drivers-260.19.36 via Portage as that version is not in the Entropy Production Repository yet. That way they (kernel and driver) should be matched. Then I'll delete ~/.kde4/ again and have another go at getting KDE 4.6.1 running.
EDIT2: Nope, no joy. If Fluxbox works fine, and apps such as XBMC and VLC play all my videos nicely under Fluxbox, it seems unlikely to be a video driver problem.
EDIT3: Filed a bug report: 2289