By MS helping to implement parts of Mono, they have, at a minimum, given up their rights to sue over those portions with which it helped. IANAL, but I have worked in the world of IP for a few years and from what I understand, MS has surrendered its rights to sue over those portions of Mono because of promissory estoppel. That is, since Mono was being implemented with help from MS, thereby giving the project its blessing to continue, MS has essentially given the developers of Mono a promise that it is OK for them to continue on.
What remains to be seen is whether or not MS would be allowed to sue for those portions of Mono that were implemented without help from MS. As usual, software development makes established IP law far more interesting to apply...
It has already been done.
"Gravitation cannot be held responsible for people falling in love." -- Albert Einstein