How to drop a NOT NULL column constraint?

I have a table created like so:

CREATE TABLE foo (
id INTEGER NOT NULL
...
);

I need to alter the table to get rid of the "NOT NULL" constraint on the
'id' column. Can I do this using ALTER TABLE? Or do I have to
dump/recreate/reload the table?

I'm looking for something like "ALTER TABLE foo ALTER COLUMN id DROP NOT
NULL", but the docs don't hint at it...

Regards,
Ed Loehr