Think stream, not synch
FWIW, I don't understand why most aTV users are so obsessed with how much content their aTV can hold or how much can be directly attached to it via USB, eSATA, etc. Who cares?
Think of the aTV as a content router. You need content from the internet (like podcasts, utube, Flickr, iTunes) - it gets it. You need content that you own and keep somewhere else - it gets it. A router isn't cursed at because it doesn't store gazillions of gigabyes, it's judged by how fast or accurately it gets the content. The same should apply to the aTV. Personally, I'd prefer the aTV not to be able to store any content locally, in which case the hard drive would only need to be as big as the OS on which it runs (pretty small), plus a little space for rented content to reside before being consumed. Whatever money Apple could save on the hard drive, they should add back in the form of a gigabit ethernet port or better wireless, as this is where your owned content comes from and the faster/better it is.... well, the better.
So if you have tons of content, great. Keep it wherever it is already located. On your iMac, great! On your Mac Pro with 4 1TB drives - awesome! On your Mac mini which is connected by Firewire to some mega-external drive enclosure or enclosures - really cool!
Not everything belongs in one place. Do you think storage on mainframes is internal to the machine? Hell no. It's somewhere else. So why does the aTV need a ton of internal storage or directly attached storage. I would think the storage should be attached (directly or indirectly) to the server (in this case, wherever iTunes is running), not the router.