This question was asked on the PostgreSQL novice newsgroup recently and Tom Lane fielded the question. Its a common thing to want to know if you are using the new 8.3 Enum feature. So we felt it useful to restate it.
Question: Given an enum, is there a query you can run to list out all the elements allowed by the enum?
Answer: Yes.
CREATE TYPE myenum as enum ('red','green','blue');
SELECT enumlabel
FROM pg_enum
WHERE enumtypid = 'myenum'::regtype
ORDER BY oid;
enumlabel
-----------
red
green
blue
(3 rows)
http://www.postgresql.org/docs/8.3/static/catalog-pg-enum.html