Maybe it's just an SSMS problem then. I didn't have any trouble creating the table and the default via SQL:

CREATE TABLE testfunc (checkdate datetime)

ALTER TABLE [dbo].[testfunc] ADD CONSTRAINT [DF_testfunc_checkdate] DEFAULT (dbo.fn_get_pst_datetime(DEFAULT)) FOR [checkdate]

GO

But if I then try to open the table in "design" mode via SSMS, I get that severe server error.