Interesting thing is that even if it's the end of the line for Microsoft developing new versions of Windows(and I don't think it is), there is still an entire ecosystem that won't be going anywhere.

I'm working for a client right now writing a few apps (away from Access/Excel type apps) so that they can upgrade to Windows 7 by mid-year 2013. And they're upgrading from XP.

So even if Windows 9 never comes out, I would guess there is plenty of work for Windows developers (for better or worse.)