By default WPF 3.5 does not come with a date time picker like winforms.
However a date picker has been added in the WPF tool kit produced by Microsoft which can be downloaded here. I guess it will become part of the framework in a future release.
It is simple to add a reference to the WPFToolkit.dll, see it in the
tool box and distribute with your application by following the
instructions on the website.
Before this was available other people had created 3rd party pickers
(which you may prefer) or alternatively used the less ideal solution of
using the winforms control in a WPF application.