Actually, with their latest console, it seems to be a little more complicated than that.
Essentially, people at Neogaf have been breaking down and analysing the Wii U GPU, and it looks like it has a Wii GPU integrated into it. Not bolted on the side, but actually integrated into the GPU itself. Imagine if a GPU got broken down into smaller bits, and then those bits were carefully placed inside a larger GPU like blocks of lego. Apparently, that's what Nintendo seems to have done. And potentially it means that not only does the Wii U have perfect backwards compatibility, but the actual Wii components, because they're integrated into the hardware itself, can be used to boost the performance of the GPU.
See, to my mind, this seems like the perfect way to go about BC. Don't just stick your old hardware on the side of the motherboard where it can sit generating extra heat. Break it down into its smallest components, then integrate it into the circuitry of whatever console you're working on now. While the PS3 and 360 are more advanced than the Wii, they're still primitive tech by today's standards. I really don't think it would take Sony that much more effort to strip the PS3 down to its skeleton, replace some of the bulkier parts with smaller, more efficient circuitry, then assimilate it into the A-10 APU they're allegedly using for the PS4. Not only would you get perfect backwards compatibility, but you'd be able to give the hardware an extra performance boost using cheap technology.