Catalyst Display Driver 7.7 and higher
When a digital panel is connected to a Radeon graphics card between Radeon models 9500 and HD2900 (including X300 through X1950) you will see many errors in the Windows event log listed as "CRT invalid display type"; with the source being "ati2mtag".
AMD/ATi has known about this issue since the end of August 2007 and has supposedly been investigating it. I don't know that they have discovered a fix yet as I just downloaded the newest drivers from last December and have these errors popping up in my own event viewer (hence my own Google chase.)
I figured I would try to save others some trouble. Click the link above to get updated information if/when it arrives. At the time of this posting, the article has been viewed 18227 times and only modified once, the day after the original post. I guess they haven't figured it out yet. :)
Their solution is to install Catalyst version 7.6; and they say it won't cause issues to the system or performance (though from my Googling, it sounds like many have had system hangs/random reboots/screen blankouts because of it.) Your mileage may vary. I haven't seen any reboots myself.
Please comment on your experiences with this issue and any better solutions (if any) you've found.