I sell a C#/WPF application (targeting .net 3.0 at the moment) and people keep asking me for a Mac version.
The application is a time tracking application with a goo
You want a new feature (support a entire different platform) and you wish to put no effort in it. Sorry, most of the time that will not work.
Besides the most basic (native) .NET stuff with mono, I do not think you can run a WPF application natively on MacOSX.
If you think you can make more money by gaining Mac users. You might consider using other presentation methods. Due to your current 'investment' I'd say try Silverlight (yes, I read your rant about not doing so). Another option could be Adobe AIR / Flex.
If you port your application this way. You can maintain a single codebase.. one that runs on both platforms.
If you wish to spend more time on new features in your current codebase: ignore the Mac users.