Here we go:
So they have officially (by positive statement I assume) said that it is:
"It is important to note that, under the Community Promise, anyone can freely implement these specifications with their technology, code, and solutions. "
So mono shouldn't be a problem anymore. Not since 2009. They even end their statement with:
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.