Cascading Updates and Deletes, introduced with SQL Server 2000, were such an important, crucial feature that it is hard to imagine providing referential integrity without them. One of the new features in SQL Server 2005 that hasn't gotten a lot of press from what I've read is the new options for the ON DELETE and ON UPDATE clauses: SET NULL and SET DEFAULT. Let's take a look!
I guess it is worth to mention that it is possible to assign a default value that is not in the set of primary keys but it impossible, after that, to insert a record with a default value. From another side you could not delete a record in a parent table with a primary key equels to default value in case if you have a referenced record in a child table.