Although the new iMac Core Duo does extremely well at H.264 playback, it apparently does so without the help of the Radeon X1600. OS X does not (yet) use this GPU's H.264 decode acceleration capabilities.
While that is a bit of a disappointment, there is a silver lining. It means that Core Duo CPU alone is very well-suited to this task, and given the fact that HD H.264 playback usually uses less than half of the computing power of a mid-clocked Core Duo, it may bode well for a future single-core Yonah-based Mac mini for use as an HD HTPC. Even if the first Yonah Mac mini is borderline for this usage, it certainly will be a vast improvement over the current G4-based Mac mini.