It's clear that this won't be desirable for every app--far from it. But from a development as well as a licensing standpoint, there are many many cases where building one app to cover a business process that spans all three devices will be hugely important for developers and customers.
There are a number of things that must be straightened out before we can assess what this killer app scenario will look like. A big question is simply how well will Windows 8 support different deployment options: Partners at WPC were assured that when Windows 8 goes GA in October. it will support multiple screen sharing scenarios. The version on my Samsung tablet has pretty much the same four scenarios available in Windows 7, with some contention between what can be on which screen when. Also necessary will be clarification about whether a native Windows 8 app can simultaneously have a desktop keyboard and mouse user experience and a tablet touch experience: I tried to see if any of the Windows 8 apps on my Series 7 would support that kind of operation, and the answer seemed to be no.
While these questions may seem trivial, the underlying question they beg isn't: How different can the Windows 8 experience be from what we think of as a desktop/tablet/phone experience today? I can imagine a number of scenarios where I'd I want sit at my desktop with my tablet mounted next to or above my keyboard and mouse, and alternate between the two input modes. Add the phone on the other side of my keyboard--maybe that's what stages all the voice communications for my Windows 8 apps--and now I have a multimode experience that, from the developer's standpoint, can be built as a single app and used by a single user across all three devices.
The bottom line for enterprise applications is that there are many processes that could span the three main enterprise devices, and the customer experience would be all the richer for it. That richness should translate into opportunity for Microsoft and its partners, and headaches for Apple and Google.
But to make this happen, Microsoft must show the world what the killer enterprise app experience can be. Maybe some partner will do it first, but my sense is that this should be job No. 1 at Microsoft. If Microsoft wants the market to tilt in its direction, it must do this. Apple had the easy job of defining the ultimate consumption device. To Microsoft falls the task of reinventing consumption to include creation.
Did I say this rivals the game-changing precedent of the Windows 3 launch? If Microsoft succeeds, that prediction may prove to be an understatement. A very big understatement.