As so rightly stated in http://www.microsoft.com/technet/prodtechnol/sql/70/books/inside6.mspx
"Currently, UDDTs don't support the notion of subtyping or inheritance,
nor do they allow a DEFAULT value or CHECK constraint to be declared as
part of the UDDT itself. These powerful object-oriented concepts will
likely make their way into future versions of SQL Server. These
limitations not withstanding, UDDT functionality is a dynamic and often
underused feature of SQL Server."
There's no point bothering with using a UDDT in T-SQL if you can't apply constraints to the UDDT!
1 vote for the introduction of the mentioned features into T-SQL
comments anyone

Introduce Domain functionality to UDDT
DaCracker_2005
Hi Kevin your link references SQL 7.0, which is almost 10 years old :) Much of this can be accomplished in SQL 2005.
The Saint
heroesch
Geert_
Sorry I'm having a lot of trouble finding quality information on how to use a UDDT as a domain in T-SQL. I thought the above was why, instead it must have been my inability to search properly.
I just found the concept applying a RULE to a UDDT which should help...
Thanks Greg, at least now I know I should keep looking.
rothco
T-SQL UDTs still lack this functionality, and rules are being depricated and won't be around very long.
If you are willing to use the CLR, the sky is the limit, but it is not altogether easy or straightforward how to use them. Start with the 2005 books online, under UDTs [CLR integration].