Can I un-jailbreak an Apple TV? I'm considering switching out my Windows-based HTPC for an Apple TV unit that will stream media off a Mac in my house. This will work out okay for me as long as I can jailbreak the unit and install Plex on it. I may just opt to transcode my existing media library but I'd like to explore both options before I decide on a best solution for me.

If I jailbreak and don't like the Plex setup, can I un-jailbreak it and get it back to factory specs? How does one do this?
Yes, doing a restore, similar to how one does this on an iOS device, will reset it to the factory default settings and install. 

You do this by going to Settings -> General -> Reset, and selecting restore. 

If that doesnt work, you can also plug it into a computer with Micro USB, and use iTunes to restore. 

More information is available in this Apple KB.