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