I had previously jailbroken my Apple TV 2G with PwnageTool on 4.0 software. Everything worked great, and I had Plex up and running. Since it appears that the most recent version of Plex requires 4.1.1, I finally decided to upgrade.
I upgraded to 4.1.1 with Seas0nPass. Everything completed successfully, but when i tether booted the Apple TV, I could not get any WiFi or Ethernet connection. Nothing is listed in the available network list, and if I put in my SSID, I get an error. If I use ethernet, it doesn't sense the connection even though the cable is solid (and the lights are on).
I restored back to official 4.1.1 firmware (8C154) via iTunes, booted up the Apple TV and confirmed that everything was working the way it should (minus the jailbreak of course).
I then re-jailbroke with Seas0nPass, and again, I have no network - either WiFi or Ethernet.