In my spare time, I wrote a MonthCalendar and DatePicker control for Silverlight. It was just a side project for me to learn Silverlight.
My MonthCalendar and DatePicker can be downloaded here. The sample works with Silverlight 1.1 RC Alpha Refresh (Sept 2007) that can be downloaded here.
Please note that the code snippet is provided under the Microsoft Permissive Liceanse, and I provide it "as is" without warranty of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.
If you actually use it.. I'll be interested to know where and how! :)