Regarding the 40gig hard drive, I do think it is an odd amount of hard drive space. I'd just assume it have only enough hard drive space to run the OS and it be a streaming machine. If Apple TV is indeed going to have space to 'move' your content in advance of viewing/listening, it needs a whole lot more than 40gigs.
My 2 cents.
The hard drive is intended to store a few videos and movies. While I'm sure that you could sync a few music playlists on the hard drive, it's main purpose is to house a few movies on it to reduce the amount of bandwidth required to stream videos. Steve Jobs basically said this at the Keynote.
Imagine, if you would, trying to stream an iTMS movie while someone else is on the network downloading a few torrents (or something). How much tolerance would you have if the movie you're trying to watch skips and glitches out. Probably zero tolerance.
So you sync the
TV to store a specific files in a "smart playlist" sort of way. Let's say that you want it to store locally the last 4 episodes of Heroes, any unwatched Jerrytime podcasts, and the last 10 movies that you've bought and haven't watched. So when you're ready to watch, it's all there waiting. Don't have to worry that someone else is tieing up the network bandwidth with a huge file transfer.
Music is different. Streaming music uses much less network bandwidth than movies and videos. You may be able to sync your music to the
tv, but I suspect that it wouldn't be quite as useful.
As for not having your library available if the host Mac is off, well, that's a problem. However, I would expect that Apple would have thought about this and has a feature to wake a sleeping Mac/PC if the
tv needs to access content. I guess a small XML file on the
tv would be stored (and sync'd in the background) that kept track of which host Mac/PC had a particular file.
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