I wouldn't see Nintendo working with Microsoft or Apple. With both of them having their foot in the gaming world, they might consider their Intellectual Properties at risk.
Google is too obvious, but I doubt it will happen. I think Google has ambitions towards an Android-compatible gaming system on the way, to follow Apple's iPhone.
Maybe they could try a large internet company that does not seem close to entering the gaming market (Yahoo?) or perhaps something like Steam, not owned by direct competition?







