Gaining remote access to your Apple TV box
After applying the patches (use the ATV patchstick creator and follow instructions found here) connect to the AppleTV box from the command line with ssh, or with a client software such as Fetch (CyberDuck does not support ssh1, FireFTP is not availbale for Firefox 3.6...).
host: appletv@local
user: frontrow
password: frontrow
First you'll need to create a new folder "QuickTime" inside ~/Library (no need to touch the system's Library folder). Then if you have Perian installed on your desktop, simply copy over the file "/Library/QuickTime/Perian.component". Now reboot your Apple TV box (using the Remote, press on the Menu button while holding the "-" button on the wheel until the system reboots).
Why?
The Apple TV will only play media available via iTunes on another machine, or purchased from the iTunes store. You may stream it, or sync the content locally. In either cases you'll only be able to play content supported by iTunes.
Perian enables QuickTime application support for additional file formats and video and audio codecs. But even though you'll be able to play avi, divx and other type of media with QuickTime, it is still necesseary to "fool" iTune. One way to that is to repackage these files in a .mov "container".) This means you don't need to actually "transcode" (convert) the files...
The best tool I found for doing that is iFlicks.app, but you can also use (free) scripts like the Apple TV Fooler. Mind you, iFlicks will also update the meta data for movies and tv shows, and is nicely integrated with iTunes.
With Perian codecs installed on the Desktop and on the Apple TV and with an "Apple TV fooler" solution you are now free to watch Divx/Xvid and many other video formats not supported by Apple.