I had just restored my Apple TV to factory settings when I realized that I'd have to transfer 20GB of videos back to it from my MacBook Pro. Waiting for paint to dry would be more fun than waiting for all 20GBs to transfer using the AirPort Express's 802.11g connection. Or, for old times' sake, I could bring the WRT54G out of temporary retirement and connect the two together. But I didn't want to do that.
as all modern ethernet-equipped Apple products are of the gigabit ethernet spec, I could just plug an ethernet cable directly between the two and party like it's 1999. And if this was 1999, I wouldn't be able to do this without a crossover ethernet cable. Update: As pointed out in the comments, the Apple TV actually doesn't have a gigabit port -- it's a 10/100 connection -- but both the Mac and ATV ports support auto-sensing, which removes the requirement of the crossover cable when connecting the two devices.
To begin, you may want turn off the AirPort connection on your Mac to avoid any network device priority issues. After connecting your Mac directly to your Apple TV, head on over to "Settings" on your Apple TV and select "Network." Then, hop on over to "Configure Ethernet" and select "Use DHCP." Voila! You may now sync to your heart's content.
Neat trick if you need to transfer a lot of files fast. Of course, it'll use the port up on both computers, and you won't have the use of your internet connection during the transfer, but for a quick share, it's the easiest way to do things.