Yeah, as an indie developer myself, I'm concerned about Windows 8. It's the first step to phasing out Win32, the framework of just about every piece of Windows software in existence today. In fact, even developing C++ Metro-compatible Win32 titles, the app-market penetration is limited only to the desktop and the Surface Pro. Neither Windows Phone 8 nor the standard Surface (which only runs Windows RT, on an ARM chipset) are Win32 capable. Worse than that, Windows 8 Phone won't even run HTML/JS Metro applications, limiting you almost solely to developing non-portable code purely for the RT platform.
Win32 likely will still be around a few years yet simply because of the overwhelming software base for it, but it'll be disappearing in likely Windows 10 or 11. It's not as simple as everybody moving to Metro, because Metro is sandboxed and doesn't allow mixed-mode applications (with the sole exception, apparently, of web browsers). Which means, no Steam for Metro. You can't make an App which launches programs, so no Metro integration for Steam or any other popular indie launch platforms.
The single biggest limiting factor to developing for Windows 8 is complexity. To develop Metro applications which work across the Windows platforms (Windows 8 & Windows RT) you're pretty much limited to using their development tools, and these are going to be more limited. XNA, the most popular Microsoft games development platform today is being phased out. Actually, it's being phased out with Windows 7. It's not supported by Metro. The Win32 applications generated by it will still run in Windows 8, but you can't produce Metro applications with it. There's a project called MonoGame which produces valid Metro targets for VS2012 from XNA code, but the "free" edition of VS2012 is going to be the most restricted in years.
In short: It's going to be much more restrictive and much more expensive for indies to develop titles for Windows 8/Windows RT/Windows 8 phone. The existing tools are being locked out across the board and can't be used to develop Metro applications. The one freely accessible resource that is capable of developing Windows 8 applications - HTML/JS - won't be able to run natively on Windows 8 Phone, outside of a browser (which in turn severely limits the performance you can get out of JS).
While there'll always be alternatives to developing for Windows, able to produce all manner of COM applications, these will largely be missing the most vital component of Windows 8 - Metro integration. They still let you play in their sandpit, but you don't get any of the toys, unless you've got the time and/or money to do so. This won't matter to the major application developers.. people will still use a desktop Photoshop application, for example, and lack of Metro integration means zilch. People will still buy the major games, and will buy Call of Duty 9 regardless of whether it's a desktop or Metro.
But for indies, it's a bad sign. The relatively unrestricted Win32 access we've had for years has built the industry tremendously, but now that indies are finally making money, the major players are moving into the indie space and they have money and resources we could only dream of. Various corps have been going around snatching up as many profitable indies as they can for the last few years. Microsoft's been in on it. EAs been trying their hand at it - hell, they own Popcap now. And Zynga.. the company which just craps all over every indie developer by, whenever they see a successful indie game, they just copy it in its entirety and give it a new name .. their marketing arm is so much stronger and richer than anything we can compete with.
These are the companies which will occupy the Windows 8 "indie" space. Indies are being forced out of the market they've built up over years. The companies that help them build it up - Steam, Desura, GreenManGaming, etc, are being forced out with them.