As you most likely already know, Mono is a project (owned by Novell) to develop implementations of .NET on a variety of operating systems including Linux, Solaris, and OS X. It is for the most part a "clean-room" effort, using no inner knowledge of Microsoft's original .NET code. It's made impressive strides over the year, and has reached the point where Mono will run .NET binaries created with Visual Studio, as well as its own native binaries, and covers a wide breadth of .NET APIs.
On the legal front, Microsoft and Novell have signed agreements to work together on some parts of Mono, and Novell and Microsoft have various other agreements in place. The Mono contributors have also pointed to the published ECMA specs for C# as the basis for their work, and have consistently said they remain ready to rewrite any portion of Mono that Microsoft should some day assert claims against (so far, there have been no such claims).
Despite all of this, Mono seems to still be a marginal player in the open source world. A search of SourceForge, for example, reveals less than 400 projects mentioning Mono, and C# projects (on whatever infrastructure) are vastly outnumbered by others. For whatever reason, the open source community has not widely embraced C# - whether this is due to its Microsoft roots, worries that Microsoft's murky IP policies will someday make Mono an untenable platform, or for other reasons.
What about you? Have you considered moving any of your own open source work to C# to take advantage of Mono's features and cross-platform nature? Or is this a piece of infrastructure that you personally do not care to touch?