Porting Tomboy from C# to C++ seems like a step back to me. C# is a higher level language than C++ and is much easier to write good and clean code with than C++. C++ rises the maintenance burden.
Also, at the moment Tomboy’s code base is under heavy development with one WSOP project (Maria Soler Climent’s) and one Summer of Code project (mine) going on. The code base is changing rapidly, so it seems this is a bad time to do such porting.