It's a nice idea, but I don't think it is going to work
What differences could there be in the boxes? Would it just be that the Sony console has blu-ray and the M$ doesn't? And then they can have completely different controllers? So would you have motion games and then controller games...
Surely the different control methods would lead to quite a lot of fragmentation and extra costs for the consumers
I think the main problem is getting the 3 hardware manufacturers to agree on a common piece of hardware. I know M$ and Sony might be able to come to some kind of agreement, but I don't think Nintendo would play along.
I do like the idea of the common online community, but the issue is the cost of Live if all the online services worked on the boxes together, then why would people pay for live for online gaming
The other problem comes in handhelds, where you will never get Apple playing along with Android, and the issues with android being fragmentation and of course the physical controls
Also most importantly, you said less hardware manufactures when of course you meant fewer
