Please look at the DateTimePicker Intro demo in the Shared Library part of the Sample Browser. The source code for that shows the XAML for it and also what other various options do (such as IsEditable).
That sample I mentioned does show how you can edit a DateTime object and enter both the date and time portions of it. The drop-down is just a MonthCalendar (no time) right now.
However that being said, we have started designing one of our next products: Editors for WPF. That will include a more robust implementation of a date/time picker, that will support things like part-based editing, up/downs, customizable drop-downs, etc. So that would probably give you what you'd like to see.