I also think Marco seriously underestimates how many great developers still use Windows. Much as I dislike it personally, there are lots of reasons to: deep Windows experience, huge ecosystem of tools/companies/etc, games, targeting a Windows-heavy vertical, and so on.
Plenty of these people want to do more for mobile devices, too. Microsoft is in a position to give them an easy way to do that (especially given the shared base of Windows 8, Windows RT and Windows Phone 8). Will it work? We'll see, but if it fails it won't be because Microsoft failed to interest great developers If it fails, it will probably be because Microsoft failed to interest consumers.
There's a very large Windows developer community. And some of those developers have thought about "working individually, in smaller companies, or in startups, building consumer-facing apps or services", and then thought about having to learn a new language, and thought about having to learn a new development environment, and thought about having people seeing you with one of those phones (1), and decided that it wasn't worth the effort or pain.
So some of those developers are just the sort of people who might make the leap into the Windows 8 ecosystem. Microsoft has always looked after developers so I agree with fpgeek that it will be more about PR and marketing and image than the ability to attract developers. Indeed PR and marketing and reality distortion was what enabled Apple to become the dominant player in the smartphone market.
Plenty of these people want to do more for mobile devices, too. Microsoft is in a position to give them an easy way to do that (especially given the shared base of Windows 8, Windows RT and Windows Phone 8). Will it work? We'll see, but if it fails it won't be because Microsoft failed to interest great developers If it fails, it will probably be because Microsoft failed to interest consumers.