I've just re-built my machine and my Apple TV decided that it didn't have any content, because it couldn't contact my machine, so I had to resync it...
It took ages, as the machines are at opposite ends of the house and only communicate over a WLAN access point.
iTunes would transfer between 100 and 1500 tunes, then just sit there. I'd have to quit it and restart it again. I think it took 12 restarts of iTunes, before it had transfered everything again!
The WLAN was still working during this period, as I was still surfing and downloading files off the net, just iTunes seemed to forget about the Apple TV...