Now, I don't know how many web developers are going to jump all over writing Windows apps all at once. Not many at first, I'm sure. What will the fleshed-out dev story be? Will the concept of server-side code have any say in the matter? And what of Windows Classic apps? Windows 7.5 will still be living under the surface - so all your old stuff will still work (not necessarily on an ARM processor though). Windows 7.5 -- Windows Classic -- will be known, over time, as icky yucky Windows, corporate Windows. Bet on it. It'll be Windows 9 or 10 before that comes to pass, but it's the direction it's headed.
Two things to look at today, if you haven't seem them already:
Thing the first: The Windows 8 unveil video.
Thing the second: PDC is now BUILD. No, BUILD doesn't stand for anything, so don't ask. Nice design on that site. Built on Orchard even. BUILD will be where Microsoft tells the developer story. I'm assuming that they'll be streaming at least the keynote, if not also the sessions afterwards -- I hope so, anyway.
I can't imagine that they'll throw out .Net entirely... no, I can imagine it. After all, I've been writing Windows applications, services, web sites, etc. for ten years - and never a line of MFC code written, never an HRESULT interpreted (outside of the occasional pinvoke).
Regardless, I'm excited about the new direction. I've been dying to see Windows throw off some of the backward-compatibility chains for years, and if it takes the ARM processor to give them an excuse, I'm all for it.