Yesterday, I posted that the TutorialNinjas site had gotten Joost up and running on their Apple TV. So last night I downloaded the latest Joost beta and gave it a spin. At first, the program seemed to go into a never ending "downloading your channel listings" cycle so I eventually just killed the process and opened the application again--and then it worked.
You do need to run it via VNC so you can type in your account name and so you can switch channels when needed, but while its going you can switch off VNC and let it run on the TV and watch it there. Unlike VLC, it does not respond to the Apple remote.
My kids are now sitting in front of the TV this morning and watching old cartoons on Joost. It works great, although you can't fast forward through the advertisements and my 802.11g network is a little underpowered for Joosting.