Yes, the Microsoft of 2012 does release a fair amount of code under Free Software licenses.
It's even contributing directly to the Linux kernel.
That said, substantial members of senior executive management, including specifically Steve Ballmer, were key in leading the dirty tricks campaigns against all competitors, from DR DOS to Novell to WordPerfect to Linux to Netscape to Sun to Google.
Some of us have long memories.
Microsoft have fallen long and hard from their late 1990s heyday, but they still haven't exhibited the cathartic rebirth of, say, IBM in the early 1990s. There is a reason the first Macintosh ad featured the hammer thrower smashing Big Brother's image on the screen (1984), and I can distinctly recall it feeling very odd to note ~1997-99 that IBM were positioning themselves as very, very strong champions of both Linux and Open Source (they liked the OS mantra better than Free Software).
They did so, however, on the basis of a very detailed, high-level, senior-executive endorsed study into the competitive advantages of pursing just this course of action (Tim O'Reilly references this in Open Sources, it's sometimes known as the "Earthquake Document", as it literally shook the earth IBM was standing on), specifically targeting its perceived major competition from Sun (since neutralized entirely) and Microsoft (a looming and large threat, now largely castrated).
Until such a time as Microsoft comes to a similar conclusion, and changes its fundamental business practices (from embrace, extend, and destroy) in a similar way, I'll continue to keep them on my enemies watchlist, if not necessarily at the top of that list. There are most definitely other entities I consider to be bigger threats, both inside and outside the tech landscape, today.