i just set up a harmony 550 with my appleTV - works great. only thing i'm havving issue with is the power off/on/toggle command, but i'm sure i'll figure it out with a few revisions.
gordon how did you pair your harmony 550 with the appletv? mine simply will not pair. i could really use your help since we use the same harmony model remote.well, what-ya-know. holding the play button on the harmony does put the appletv to sleep/off (can't tell which).