In truth, the biggest factor in how this new machine will affect Sony and Microsoft is likely to be the attractiveness of the USP. The probable easy porting of content will level the software playing field as far as multi-platform titles go, but for exclusive content, would certainly give Nintendo the advantage in that area.
Touch-screen control is interesting, as we have seen with Sony's NGP demo's, and it is proven as a popular control mechanism, not least with Nintendo's own DS, but is that a game changer on it's own?
If the USP is streaming content to the handsets, what kind of content and how will it be implimented? For instance, games are an obvious starting point and naturally the focus in discussions, but presumably internet access will be streamed to the handsets too, what about media players for online music and T.V. or even films? Why not, these are standard console functions nowadays, so if streaming is the "big thing" then the Cafe will likely stream all these functions too.
This could be a massive selling point, above and beyond the abilities of the other two consoles. In addition, making the cost of each additional handset much less of a barrier to sales. If you could get all that functionality for another family member anywhere in the house, what would you pay for another handset?
All pie in the sky at the moment, but I wouldn't bet against Nintendo changing the game again.