I just don't think there's any reason to jailbreak and old Apple TV to get it to work with SMB and a few extra codecs/containers when retail products already exist that do all that for a lower price. WDTV Live for instance.

Plex is another matter entirely, but it will probably perform a lot better on a micro-sized commodity PC platform than on an ATV. Some people have high hopes for Raspberry Pi with Plex or XBMC. I won't hold my breath for that one.