In kubuntu Lucid with kde 4.5.3 my Kdm ( login screen ) goes to standby after
10 minutes. In system settings -> power management -> I turned off standby,
suspend, etc).

In xorg.conf:
Section "Monitor"
# HorizSync source: edid, VertRefresh source: edid
Identifier "Monitor0"
VendorName "Unknown"
ModelName "LG Electronics 500G"
HorizSync 30.0 - 54.0
VertRefresh 50.0 - 120.0
Option "DPMS" "false"
EndSection

Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce 7025 / nForce 630a"
Option "DPMS" "false"
EndSection

$: xset q
DPMS (Energy Star):
Standby: 0 Suspend: 0 Off: 0
DPMS is Enabled
Monitor is On

But kdm still keep going to screensaver/standby.

I have this same problem, does anyone have any ideas?

If you can use commands such as xset, etc. then you should be able to edit /etc/X11/xdm/Xsetup or /etc/X11/xdm/Xsetup to get those commands executed on X startup (assuming X terminates and relaunches once the user has logged out).