Plans are for at least one version of the box (how can you call anything that slim a box?) to run Android.
[Nicholas] Negroponte also stressed the importance of the open source nature of the project, pointing out that every aspect of the the project would still be open source. "We're going to go totally open," he said. "In some sense it's the complete opposite of the Amazon bookstore or iTunes, where we'll run anything, including viruses and Flash."
(Weintraub link via The Code Project.)