Got my replacement iMac yesterday having backed up the old one as a disc image, and set it restoring last night (which took about 5 hours) and left it in FireWire Target Disk Mode over night. I restarted it in the morning and all was well and good. I left the new iMac on all day to see if this one shared the problem, and when I got home from work it woke up flicker free after waking the machine from a deep sleep.
So this evening I set a DVD burning in Toast (7.0.2 - PPC), ran an installer (Parrallels Desktop) and went and had my dinner. I couldn't have been away for more than 90 minutes, but when I got back to the iMac I woke it up and the flicker had arrived and on it's second day!
http://discussions.apple.com/thread.jspa?messageID=2487667#2487667