Simple aggregate query brain fart

I want to do:

SELECT id, count(*) FROM mytable WHERE count(*) > 2 GROUP BY id;

But this doesn't work because Pg won't allow aggregate functions in a
where clause. So I modified it to:

SELECT id, count(*) AS cnt FROM mytable WHERE cnt > 2 GROUP BY id;

But Pg still complains (that column cnt does not exist). When using an
GROUP/ORDER BY clause, I can refer to a column number (e.g., GROUP BY 1)
instead of a column name, but how can I refer to my unnamed second
column in my where clause?

Mark