Kinda late now but…
10.6.5 killed my external monitor support on my iMac. I didn't know until now because my iMac has been in the shop for 2 months. They finally replaced it with a summer 2010 iMac Core i7.
The external monitor works fine on my 10.6.5 MacBook Pro, but not on the 10.6.5 iMac. I have confirmed that monitor works fine in 10.6.4, on two different installs. The iMac comes with 10.6.4. With that loaded, the external monitor works perfectly. I also have the old drive from my now defunct old iMac and that had 10.6.4 on it. I booted that up, and the external screen works fine.
However, that same 10.6.5 drive that worked with the MacBook Pro doesn't work with the iMac. Blank screen. I tried re-installing the 10.6.5 update with no resolution.
It turns out there is a relatively commonly reported screen flicker issue now, specifically with 10.6.5, reported on the Apple Discussion forums.
http://discussions.apple.com/thread.jspa?threadID=2644371&tstart=0
They get flickering, but I get a completely non-working monitor. Well not completely. The iMac thinks it's there, but the monitor is just blank.