Microsoft has opened up ASP.NET MVC under its Microsoft Public License, a valid open source license. As Miguel de Icaza says, Microsoft contributing to open source is a good news for everyone. And with Mono, ASP.NET can go cross platform as well.
Microsoft seems to be seems to be starting from the opposite end than most of the open source projects. First Oxite – a CMS built on ASP.NET MVC – was open sourced, now the underlying MVC stack. I hope the trend continues and Microsoft opens up more core components.
Microsoft has to participate in open source, not only to compete with the likes of Linux and OpenOffice, but also with Mono, .NET’s open source counterpart. You might notice that with Mono and its tools, like MonoDevelop, might provide a much better open source ecosystem than .NET itself.
I look forward to more contribution from Microsoft to open source, not only by contributing code, but by also encouraging and acknowledging good open source work happening elsewhere.