I think I'm seeing this too.  Mac Pro (2009) with GT120 (G96/NV50) and 24 inch
Apple Cinema display via MDP connection.  powersave blanking used to work in
3.6.3 which was the last kernel I used (opensuse 12.3 otherwise).  Now it does
nothing in opensuse 13.1 which has 3.11.10, also notwork with a 3.14.1 kernel. 
X thinks that it's turned off the display (according to xset q) but nothing
happened.

I would be happy to test a fix if and when a trial is ready.  I have all sorts
of trouble with nouveau on this hardware so I suspect it's not a common setup. 
:-)
Fortunately, recent versions seem pretty good other than the lack of powersave.