I finally got the chance to test the iMac Core Duo 2.0 GHz myself with HD H.264 QuickTime playback. Contrary to numerous claims online, all was not perfect. The iMac exhibited significant stuttering between in a few spots in the 1080p The Macaulay Library clip between 1:03 - 1:15, with frame rates dropping to as low as 16 fps. Granted, this clip is especially hard to decode since it has a 30 fps frame rate and the full 1920x1080 resolution, but it was still a bit disappointing to see this.
However, CPU speed may not be the limiting factor. According to OS X's Activity Monitor, CPU usage for this clip never went above 145% (out of 200% for two cores). Indeed, when the same clip was played back on a Quad Power Mac in a window, once in a while there was very slight stuttering in the same spots, although it was inconsistent and there was never any stuttering when the clip was played back in full screen mode.
However, on both machines, other 24 fps HD clips I tried played back perfectly.