Any of the third party hacks by what I understand allow show extraction to a PC, and the ability for the Tivo to use ethernet instead of a phone line for guide updates. Since they don't modify the core Tivo software, they can't allow one Tivo to stream to another.

The ReplayTV units indeed only stream shows to each other, but not music off a computer. You can put photos on the ReplayTV units to see them on the TV, I personally haven't used it yet, so I can't comment.

Oh, one nice thing about ReplayTV units still. The 5500 does lack the automatic commercial skip, but the technology is still in the software running. So, to skip an entire commercial break, you hit the right arrow once. This works on any ReplayTV unit, not just the one the show is sitting on.