By the time the PS4 rolls around the Cell processor will be so inexpensive that they could throw 3 or 4 of them into a console at the same cost as a PS3 is now. I don't see this addon coming to life just because it's so impractical.
The PS3 can already do everything, it only needs to do it better at this point.
Here's how the PS4 should go.
Multiple Cells
Throw out the GPU entirely
Add more XRAM
Faster BD Drive
Wireless N
USB 3.0
The cost of the Cell is going to drop real quickly what with it being put in TVs and what not now. So Sony can utilize a multiple Cell architecture that requires no GPU. Simplifies the hardware, allows for smaller chipsets and maximum performance. I believe this can all be done at a relatively low cost, with some fantastic results.
