Versions tested :
xorg-x11-drv-nouveau-0.0.12-31.20090421git47bb00f.fc11
xorg-x11-drv-nouveau-0.0.12-32.20090501gitf69b34a.fc11

Hardware :
01:00.0 VGA compatible controller: nVidia Corporation GeForce 8500 GT (rev a1)
Monitor connected on HDMI.

http://www.smolts.org/client/show/pub_3bfafa70-b96a-4f6f-8c85-b26e2fcc7eee

Monitor goes very quickly in in standby mode, after only 1' of inactivity.
Of course screeen saver is configured (20') and energy mode (never).

From Xorg.0.log (entering in standby mode)
(II) NOUVEAU(0): nv50_output_dpms is called with mode 3.
(II) NOUVEAU(0): NV50SorSetPowerMode is called with mode 3.
(II) NOUVEAU(0): NV50DacSetPowerMode is called with mode 3.
(II) NOUVEAU(0): nv50_crtc_dpms is called with mode 3 for CRTC0.

From Xorg.0.log (end of standby mode)
(II) NOUVEAU(0): nv50_crtc_dpms is called with mode 0 for CRTC0.
(II) NOUVEAU(0): nv50_output_dpms is called with mode 0.
(II) NOUVEAU(0): NV50SorSetPowerMode is called with mode 0.
(II) NOUVEAU(0): NV50DacSetPowerMode is called with mode 0.

Don't find any information about what causing this or about an configure option.