Well, since installing Leopard didn't work out for my ideal download box (randomly restarted every few minutes), and I hadn't made a backup of the ATV HDD before I wiped it (tried to, but did something backwards and lost it), I was sitting with a box I couldn't use for 2 weeks - same as a lot of you!
I figured out a way to get your ATV back to the ATV OS, which I think is what a lot of you want to do!
There might be a way to do with without taking out the HDD, but it wasn't a problem for me to just pop the HDD out and format it.
Download this - AtvDrive.
Format your ATV HDD with it.
Download Firecore's ATV Flash via buying it or finding it at your favorite spot. I'd suggest buying it if you want to use their added features, if not, you're just downloading the latest ATV OS using their software. Up to you.
Start up ATV Flash, agree, next, ect., blah, blah, blah, download the OS.
It'll download the ATV OS and should save it to your Downloads folder.
Download Carbon Copy Cloner.
Mount the ATV OS in Downloads - Should be something like: latest1304246102600.dmg
Clone it to the OSBoot partition on your ATV HDD.
And... you should be good to go!