Firstly, I would suggest that the recent trend of judicial decisions in patent law has been to offer some form of reward for hard work. This might (and we are talking mights here) be enough for the award of a patent to Ximian for Mono (assuming they would want it). Also, the fact that Microsoft are only developing DotNet for the Windows platform might provide a ground for limiting their patent monopoly to DotNet on Windows. If not, then it might be possible to get a compulsory licence on the basis of Microsoft's failure to exploit the DotNet 'invention' on the Linux platform.

Secondly, competition law must have a role. Microsoft would have to be careful about running afoul of antitrust legislation in the US (again).

Finally, it would be interesting to see how many countries Microsoft have been awarded patent protection in. Maybe Mono could base itself in somewhere like Vanuatu :)