Tethered jailbreak on windows does work. I did it last night on Windows 7.
It isn't easy though.
Firstly, you need your IPSW from Pwnagetool, itunes 10.1, and download iReb and windows tethered boot. I will provide links if needed.
Once the IPSW is produced, you don't need a mac at all.
It is a pain in the behind to drop the ATV 2G into dfu. Note that if you try and restore in Recover mode it won't work - you will commonly get error 21.
iReb can help with timing to drop into DFU - I found it very useful.
If you aren't using iReb, make sure that you have unplugged the mains power. and started iTunes.
2) Then, with the USB plugged in, hold down and menu for 6 seconds until the light blinks quickly, then immediately release and then hold menu and play/pause.
You should hear your Windows PC make the usb (dis)connecting sound two or three times,
Windows should install the DFU drivers.
Restore your Pwned custom IPSW using iTunes.
Now copy the IPSW, and rename it with a .rar extension.
Drop to DFU again. This may take a few goes.
Open this rar, and copy the kernel.release.k66 (or something like that) into the same folder as tehheredboot for windows.
Do the same with the IBSS file - you may need to drill down a few levels to find it.
Open a command prompt, adn change directory to the folder where your tethereboot lives along with the files you just extracted.
Making sure you are still in DFU, run the following in the command prompt:
tetheredboot -i iBSS.k66ap.RELEASE.dfu -k kernelcache.release.k66
If all is well, tethereboot will exploit the AppleTV, upload the kernel and iBSS files, and it will boot tethered.
Now do your aTV install, restart, run a tethered boot from dfu again, and it works.
EDIT: If you get error 1600, then you can delete some iTunes files to resolve this. Can't remember the exact path, but post here if you get this error and I'll find it for you.