I don't think it's good idea for multiple reason. I think that Ms is already likely to lose money on Natal, they will have to subsidize, with new hardware they would likely have to subsidize too if they want to stay cheap. I don't thinking that the saving done on Natal by removing say a 60mm² chip @90nm (thus produced cheaply) would make up for that. Not to mention that a well design chip/DSP can make quiet some work so we don't know how many "general purpose" horse power would be take away from the system.
Gamers reaction would be horrendous and after the first XBox disaster, the RroD MS doesn't really further bad publicity.
Imho should push out a really flawless 360 slim, pack in 16 or 32GB within the system (I would favour 32GB) and sell it at the same price as the current arcade pack. That would be a nice upgrade as Ms would no longer have a "sub part" (or percieved as such) system to sell. Using 45 and 40 nm part Ms may even make some money.
They should push the Natal 360 @299$ (same slim 360 as above + Natal) The few money made on the slim may offset a part of the loss induced by Natal pretty cost (I mean Ms may slightly more that 120$ to cover Natal cost).
Ms may try to regain more money with the sales of separate HDD for both remaining system in their offering. Every system sold would have what it takes to bring them extra money through the various services offered by the Live paltform.