Microsoft is saying to its developers, "We can give you a nearly pain-free path to ease into the mobile world; you don't have to throw away all your code or relearn everything you know." Best of all for Microsoft, it preserves their investment in existing technologies like .NET, doesn't actively cannibalize the profitable desktop or server businesses, and fits in with Ballmer's promise at CES: "Whatever device you use, Windows will be there."
Developers already engaged in mobile app development on Android and Apple platforms will not find this argument convincing, though. Microsoft has almost no presence in the mobile world today. With Microsoft so far behind, it won't make sense for ARM Windows to be the tail that wags the dog as far as choice of mobile development environment goes.
One thing that seems to be missing from this strategy is consumers. Microsoft is hoping that if they can create an environment where consumers can use at least some of their existing devices and software with Microsoft mobile operating systems, they will be more likely to go with Windows. I am not so sure about this "Build it and they will come" approach.
Consumers already have plenty of non-Microsoft options like the iPad with sexy hardware and a decent selection of apps. Microsoft's odds may be better in corporate environments, where in-house familiarity with .NET may possibly drive the decision to use Windows-based tablets and phones.