I have a browse dialog and want to show a datepicker in but it's only showing as a textbox. I have made sure that the field in the database is of datetime and that is was mapped successfully in the business object. I have tried it on another browse dialog with the same results. Any ideas?

Ben Hayat is correct.  The GetType() does not return a DateTime but rather a Nullable data type....thus you will never get a DateTime field.  You are better off using the Return Alternate Value on Null rather than Nullable within the Customization so that the data type remains a DateTime but allows for a NULL exception.  As long as you have your field typed as a Nullable (or use Nullable Generics within the customization) the BrowseDialog will never see that property as a DateTime.