If you're a consumer, you use the Metro UI.
If you're a producer, you use the Desktop.
The line between "consumer" and "producer" is so fuzzy that this doesn't sound feasible to me. [For the record, I haven't used Windows 8 yet.]
Both "types" of users—if we assume for argument's sake that such a distinction can even be made—tend to use a number of apps; each of those individual apps falls somewhere on a continuum of consumption vs. production. And even where each app falls on that continuum for that person may vary throughout the day!
I expect there will continue to be tablets — and they should have tablet UIs — and there will continue to be "desktops" — and they should have desktop UIs. They should be made similar in aspects where it makes sense, kept different in those aspects where it doesn't, and there should definitely be many apps that run on both (with appropriately varying UIs depending on platform) and which constantly sync so that the users' production and consumption can flow between them as desired.
If Microsoft pulls off a single system (even with two modes) that can transition without confusion and frustration between the two and that doesn't heavily compromise either one, then they'll have a very unique offering. But I don't think it's clear that such a goal is even achievable.