question re. count, group by, and having

The following query returns an error ("column c does not exist") in pg
8.0.3:

(The column 'state' is the two letter abbreviation for a US state)

-- get the number of rows for each state; list in descending order; include
only states with at least 6 rows

select state, count(state) as c from t group by state having c > 5 order by
c desc; -- gives error

If I leave the having clause out, I get the expected results:

select state, count(state) as c from t group by state order by c desc; --
this works

Is this a bug or a feature? I'm not sure why I can use 'c' in the order by
clause but not the having clause. pg is much happier with the full "having
count(state) > 5". Will this cause count to be evaluated twice?

If it matters, state is varchar(2).