Good development question!
Other World Mapper is built on C++ and Microsoft libraries for the UI. When we started development back in 2010 (as a side project), things didn't look so promising for managed code and .Net for cross platform implementations, so we did what we could to keep the core of OWM away from OS specific libraries (with the UI being the exception).
Mono is an option for us, and so are some other cross platform GUI libraries that use the OS's native API's (i.e. carbon) to give the "appropriate look" for the OS. If it comes down to it, since most of the code is C++ with cross platform graphics libraries, we'll build the GUI for Mac and Linux natively.
Alejandro S. Canosa
Three Minds Software