
If Microsoft wanted to get involved in the open source game they would have a couple options: First they could just open source Windows, Office, etc... I have a feeling this would be a major disruption to their business model.
Alternatively, they could take a much easier route: adopt Linux as their own. If Microsoft can continue to make deals with Linux distributions they would have an open source platform that is mature and well established throughout the world. If Microsoft could adopt Linux in such a way that the community support for it remains strong, the deal would work to Microsoft's benefit.
I'll give an example. What if Microsoft could extend the olive branch to a distribution like Ubuntu in such a way that the community, for the most part, would still support it? Microsoft probably sees that a well maintained community Linux distribution is fairly cheap to run. (I believe Mark has invested ~$10 Million). That's pennies for Microsoft. Furthermore, not only is Ubuntu a very solid community distribution, it has deals with major players like Sun and Dell and therefore has the potential of standing on its own two feet and making a profit in a commercial way.
Now, in reality, Ubuntu has not announced any deals with Microsoft. However, I believe the same logic applies to the distributions they have cut deals with. They are established open source platforms, cheap to maintain because of community support, and have proven they can make it in the commercial world. Furthermore, If Microsoft could could find a way to get all these Linux distributions to collaborate and work together, Linux would be a much more standard platform across the various distributions. This might actually be good for Linux and open source as well.
This would give Microsoft an open source future in a very painless way: adopt the already established Linux platforms as your own. Extend the olive branch, give them their new GPL 3 and reap the benefits of Linux in the commercial market, the future commercial market based on open source.