The ATI video chip RN50 (aka ES1000) provides digital video via external TMDS as well as standard VGA output.  The digital video is a 24 bit parallel video output plus 4 bits for control signals.  This is occasionally termed DVO - for Digital Video Out.

We have found that the Radeon driver provided with SLES 10 and RHEL 5.2 inappropriately clears the disp_hw_debug register which is used to set the output impedance of the buffers for the DVO signal.  This register is at offset 0xD14 on the RN50. By setting this register to zero the Radeon driver causes these signals to be too weak for some DVO receivers.  

The disp_hw_debug register is set by the RN50 video BIOS and should not be modified by the Radeon driver.

To verify this:  

1.  Boot to Runlevel 3.  Read the register.  It has the non-zero BIOS settings.
2.  startx, read the register again.  It is now 0.

The problem is seen on some KVM systems that use the RN50 digital output.  Due to the nature of the signals this is only a problem for resolutions above 1280x1024.