Someone asked me this recently and not playing with custom types much, I’m not sure this is the best solution. Anyrate here it goes. Note this works for PostgreSQL 8.2 and above. Note sure about 8.1 etc. I assume it probably does.
Let us say you created a custom type something like this:

Mas:

http://www.postgresonline.com/journal/index.php?/archives/118-Creating-instance-of-custom-type.html