Just made the jump over to Natty. Fullscreen video, by VLC or flash, used to supress monitor blanking / power saving, but does so no longer.
Is it a configuration change, that I can undo? Or is an API change and that the apps are not keeping up with?
Figured it out. The screen-saver package was removed for some reason (I've been updating since v.6.04..) which apparently controls these things.
Installing it returned things to normal.