Apple's 27" iMac only supports native or 720p video input, no 1080p
Apple's revised 27" iMac introduced this spring debuted a new feature: a Mini DisplayPort video port that worked in both directions, enabling external video sources to output video to the screen. That feature is largely limited to 720p in HDTV applications however.
http://feeds.appleinsider.com/click.phdo?i=8f646aca8e72a180bc9a5ad38874f917
