I am currently developing a C# / .NET user interface platform that will finally allow you to write the UI once in C# and create:
I will consider adding Windows Phone and Silverlight, which should be easy ports.
The platform will include an attractive and flexible calendar control.
The project will represent about 500 hours of development. The components will be released under commercial license. The target price is $200-300. I hope to release many building blocks of the platform under an open-source permissive license.
The calendar component is currently in alpha release.
If you would like more information, or would like updates as they become available, please contact me.