June 20 2007 Patch (aka the YouTube Patch)
As described in the Patch Over Network article, the AppleTV goes out to mesu.apple.com perodically and checks for updates, much like Software Update on "real" OS X works.
The DMG is accessible from http://mesu.apple.com/data/OS/061-2988.20070620.bHy75/2Z694-5248-45.dmg. It weighs in at 179MB (188,072,708 bytes)
Full file listing of the disk image
What has changed in the update from the original vanilla factory image?
I'm guessing a FRApplicance for the YouTube integration.