bug? non working casts for domain

Dear PostgreSQL developer.

Although it is allowed to create a cast for a domain, it seems that there 
is no way to trigger it. You can find attached an sql script to illustrate 
the issue with postgresql 8.1.3. The create cast and create domain 
documentations do not seem to discuss this point.

ISTM that it is a pg bug. Indeed, either

(1) the create cast should be rejected if it is not allowed for domains.

or

(2) the function should be triggered by explicit casts to the domain.

Have a nice day,

-- 
Fabien.