After doing an untethered jailbreak on my Apple TV 2G (ATV2) with GreenPois0n RC6, I noticed that if I rebooted the Apple TV my wifi would not reconnect. Disconnecting the power and plugging back in would not resolve the issue. After a bit of research, I figured out a quick fix to getting my wifi back:
Go to: NitoTV > Settings > Restart Lowtide
After the device rebooted, my wifi connection re-established itself within 10-15 seconds. To permanently fix the issue, I SSH'ed into the AppleTV and issued the following commands:
apt-get update apt-get remove com.nito.nitotv apt-get install com.nito.nitotv killall AppleTV
Update (2/20/2010): A number of blogs are reporting that greenPois0n RC 6.1 includes the wifi fix directly. So if you haven't already flashed, just go ahead and use 6.1 and it should include the wifi fix. Additionally, you can always flash RC 6.1 over your current setup, but I find it easier just to either remove/install com.nito.nitotv or install OpenSSH from the nitoTV menu.
Update (2/16/2010): NitoTV recently posted on their twitter account the the wifi issue appears to be related to SSH -- they recommend that you grab OpenSSH from the top of the nitoTV featured list and reboot to fix the wifi issues. You shouldn't need to do both fixes (the commands above and the OpenSSH install), but it also shouldn't hurt anything.
On Wednesday 16th February 2011, @nitoTV said: the wifi fix is live!! thanks @saurik and @DHowett for major contributions to squelching the problem! it was an SSH issue and saurik has updated openssh to 5.8p1-9. grab openssh from the top of the nitoTV featured list and reboot and your wifi woes /should/ be history!!!
Note: If you are running a tethered jailbreak, it's recommend you update to an untethered jailbreak such as the GreenPois0n RC6 jailbreak. fireCore also has out a beta version of their seas0nPass utility. However, with the untethered Windows version of GreenPois0n being out for Apple TV 4.2.1 devices, I don't see why you wouldn't just want to use the GreenPois0n release as it's just as user friendly... well, at least as user friendly as a jailbreak can be.