Yup - exactly. The signalling rates on the XDR and FlexIO bus (connecting the Cell and the RSX) are insane (talk about several GHz). If you got the kind of equipment to pull that off then you must be a very, very rich man. The PCIe analyzer we got over at the lab costs as much as a small house. And that stuff is slow compared to said busses.
On top of that, documentation for the protocol of the XDR and FlexIO busses are only available via NDA. Yeah - the guys over at Rambus are a bunch of greedy bastards