Hi Sander,
You can use the idea presented earlier in this thread of changing custom formatting of the DateTimePicker control, depending on if the control is checked or not (DateTimePicker.Checked).
Only problem is that you don't get a Click event fired when you click on the checkbox within the control, so you have no way of knowing if it's being changed or when.
There is an issue about implementing CheckChanged event in our issue tracker here. When that event is implemented you can use is to change the formatting of the control whenever the check changes.
Palli