Look in your xf86config file and find the Graphics section. See if yours has the DPMS option like this:
Identifier "** NVIDIA (generic) [nv]"
Option "DPMS" <<----------
# Insert Clocks lines here if appropriate
It may not have that in there. Then you will need to go into the power control and turn it on and set the timings that you want.
My Gentoo did not have that option either. I had to dig to find out what made that work.
Hope that helps.
edit: leave out the <<---------- part. That was my pointer.