You should probably check out Mono and MonoDevelop. It even has the possibility to put your program into an OS X App bundle.
If you want to put it on the App Store, make sure to include the Mono runtime.
Maybe you should consider writing a separate OS X UI. You can still use C# for this, thanks to MonoMac.
Mono is OK - but only supports a subset of the .Net libraries, so unless you built your C# project with mono compatibility in mind, you'll still have some porting to do, anyway.
If you wanted mono compatibility on Linux or OS/X, you'll probably also want to make sure you are compiling wih /clr : pure.
All that said, though, depending on the code base, I'd probably just port it to C++ or Objective-C. It'll give you the best native performance on OS/X and you'll get a change to rethink the code as well. Of course, from a time perspective, this is the most expensive option.