Unix.SE is about unix and Linux (and perhaps POSIX, but that's not been determined yet). Most of the traffic is about Linux, which is unsurprising since it has the biggest market share (most Mac OS X users don't interact with the unix part). But non-free unices are equally on-topic (there have been questions on AIX, CoLinux (seems to be on-topic) (CoLinux is open source but can't be run without some closed-source software), Cygwin (same remark as CoLinux), HP-UX, OSX (they're not all about the open source BSD bits), Solaris (they're not all about OpenSolaris), non-free drivers on Linux, etc.).
Conversely, questions about a piece of software for Windows wouldn't be on-topic just because the software itself is open-source. Whether discussions of open-sourceness as relating to Unix/Linux is on-topic is not well-determined yet.
A site about open source software wouldn't make sense as a place to discuss technical issues, because there wouldn't be any unity of subject. It would make sense as a place to discuss project management, licensing, funding, marketing, etc. — and there is already a proposal for that.