Personally, I don't think so. But this is my own opinion
My argument being, why use .net (a Microsoft Product, subject to their often over-zealous licensing terms) when you can use Java to create a true multi-platform application, with the same ease as .net?
Also, C++ allows for multi-platform development, offering performance you can't have in Java, however, it takes longer to developer.
Personally, I understand why Mono exists, but it will not replace Java.