To my knowledge Mono has never been officially accepted or approved but Microsoft has stated through (semi?) official channels that they won't go after the project in a legal way. Quite frankly I think that they'll commit reputation suicide if they decided to go after it. Although people who use Mono are usually not doing that on a licensed Microsoft platform, in the end they're still using a Microsoft technology, which also accounts for something.
Anyway, as to Microsoft and Open Source Software, maybe also a fun thing to know: a few years ago Microsoft actually sat in the top 20 of most active contributors to the Linux kernel. So, not contributing money but actual code updates.
Obviously they followed their own agenda, to my knowledge most work was done with Samba and Microsoft Servers in mind, but even so the fact still stands.