While on a location shoot in Orlando my iMac wouldn't boot. Well.. I found out it was booting... no display/screen action. After 2 mornings of weirdness and safe boots, PRAM resets, etc... I caved and called Reboot. He rang me back within a very short time and attempted to find the problem. The video came on right before he called... so I was reluctant to do anything except finish the shoot.
This morning it was the same... so I booted it and went outside. Approx. 45 minutes later I came back in and the video was fine. So apparently when it warms up... it works.

It was still under a 3 year warranty, so I took it to the Apple Genius counter and they agree... either the vid card or the monitor/screen.
The tentative invoice said vidcard would be 149.00 and a monitor would be $510.00.
So glad I ponied up the $80 bucks or so for the extended.
Should have it back in 5-7 days if it's the card... and only a couple of days if it's the screen.
The vid card wasn't in stock.