I helped build a web service product that ran on Mono in the early 2000's back when I was still also a Windows dev but with the significant improvements over the mid to latter 2000's to the speed and stability of the JVM I dare say (and consider Mojang's success) the JVM would be a better choice for future game engines than Mono. With so much work having gone into using Mono I doubt it would be changed to the JVM but also Microsoft to their credit is open sourcing some/a lot? of the .NET runtime to open platforms like Linux.

The only problem I have with Mono today is Miguel's decision to completely disrespect and disregard the Linux Desktop: http://tirania.org/blog/archive/2012/Aug-29.html