There is some risk that they'll take it back closed source since its MIT licensed (with patent grant) and their CLA[1] gives them a broad copyright license. However, I think with such a high-quality implementation being open source would be hard to put back in the bottle, especially since Mono is already integrating much of the code into their runtime[2]. On the other hand, external contributions to the Mono runtime are MIT licensed so that project could jump back to closed source if Xamarin so desires.
It's important to note that all of this applies to the OpenJDK too[3]. So you are at least no worse off if you make a leap from the JVM.
[1]: https://cla2.dotnetfoundation.org/cladoc/net-foundation-cont...
[2]: http://www.mono-project.com/docs/about-mono/dotnet-integrati...
[3]: http://www.oracle.com/technetwork/oca-405177.pdf
edit: word choice