GreenPois0n RC6 is on its way to untethered iOS 4.2.1 jailbreak for your Apple TV2G as it has been confirmed by Pod2g that it'll add support for the ATV 2g in the next tool release. A tweet was released by him along with a readme file that confirms that the next GreenPois0n update "RC6" would be able to jailbreak your Apple TV 2G device.
From the Readme file:
*** greenpois0n 1.0 RC5 ***
> Description of the software:
greenpois0n will Jailbreak your iDevice.
> Supported iDevices:
- iPod Touch 2G (all bootroms)
- iPod Touch 3G
- iPod Touch 4G
- iPad 1G
- iPhone 3Gs (all bootroms)
- iPhone 4
- iPhone 4 Verizon (_not_tested_)
- ___AppleTV 2___ (support will be added in RC6)
> Requirements (check this !)
- a supported device (see above)
- the device should run iOS 4.2.1. Please check on the device in the Settings -> General -> About page.
If your device is running an older version, please upgrade iOS first using iTunes.
*** If you want to SIM Unlock, preserve your baseband using TinyUmbrella ***
- at least MacOSX 10.5
- OSX in a VM won’t work
- a working internet connection
> Notes (checks this also !)
- if your device is jailbroken with iPhone Dev Team’s Monte, please restore to fresh 4.2.1. first
- if Cydia doesn’t show up after Loader, you may need to reboot the device
- this version of greenpois0n doesn’t include a patched activation system. You’ll need to activate your device using iTunes
- WE DO NOT SUPPORT PIRACY !
1. plug the device to your computer using the USB cable
2. quit iTunes if it shows up
3. start greenpois0n.app
4. follow instructions
5. the device will boot a 1st time with verbose text on screen. This operation will installthe jailbreak on the device.
6. the device will reboot normally
7. start the Loader application (green icon on the SpringBoard), and follow instructions to install Cydia
As the GreenPois0n RC6 update is not yet available but will be after the Chronic Team releases the GreenPois0n RC5 for windows users and it'll arrive after a 2-3 day break.