Quote from the announcement by Peter Galli at MS:
It is important to note that, under the Community Promise, anyone can freely implement these specifications with their technology, code, and solutions.You do not need to sign a license agreement, or otherwise communicate to Microsoft how you will implement the specifications.
The Promise applies to developers, distributors, and users of Covered Implementations without regard to the development model that created the implementations, the type of copyright licenses under which it is distributed, or the associated business model.
Under the Community Promise, Microsoft provides assurance that it will not assert its Necessary Claims against anyone who makes, uses, sells, offers for sale, imports, or distributes any Covered Implementation under any type of development or distribution model, including open-source licensing models such as the LGPL or GPL.
Full Community Promise over at Microsoft.
Note: as Miguel says in his post, mono implements a whole bunch of .Net technology above and beyond ECMA C# and CLI and those parts (ASP.Net ADO.Net etc) aren’t currently covered by this promise.