Microsoft FFmpegInterop library for Windows
github.com
github.com
Depending on how that case finally comes back, I'd expect that could really affect Microsoft's strategy, here.
This issue has popped up a number of times for Linux kernel modules, and at what point they are a derived work - https://lwn.net/Articles/13398/. Here is an email thread concerning this http://yarchive.net/comp/linux/gpl_modules.html.
If somehow MS can't make and open source clean compatibility layers for their own software, then that's a big problem.
Is that distinct from being copyrightable?
I'm sure that if it becomes a big enough problem, the problem will be buried in lawsuits once Microsoft or IBM or similar stub their toes on it.
Remember that because there were originally patent claims in the suit, the appeal went to the Federal Circuit -- even though the patent claims were dropped -- not the Ninth Circuit where the district court is located. As a result, the Federal Circuit's ruling that APIs are not uncopyrightable is not precedent since it doesn't concern patents (the Federal Circuit's remit).
Their reasoning might influence future rulings, but precedence-wise the ruling has no bearing on the future of copyright on APIs in the US.
edit: the EFF has a little more detail:
https://www.eff.org/deeplinks/2015/06/bad-news-supreme-court...
https://www.eff.org/deeplinks/2015/06/eff-stands-innovative-...
Edit: Works Flawlessly in Linux.
It seems like this library is specifically for integrating FFmpeg with the built-in classes for video playback in Windows Store apps, but you don't have to use those classes to use FFmpeg on Windows.