I haven't taken the time to downgrade a machine to Tiger yet to try ATV4Mac, but had been planning on trying it soon. Can you answer a question or two?
1 - Can you play Video_TS files, or only movies that are shared via iTunes?
2 - Rentals? Of HD movies?
1) Anything Perian can play. 1080p plays just barely on my MBP. If the folks @ Perian could offer the drop frame tweak we use in VLC, then it would be flawless. Yes, it plays movies shared in iTunes but includes ATVFiles, a plug in to play anything via quicktime.
2) Rentals apparently work, although I did not try. Folks in the forums suggest HD Rentals have issue.