set-valued function called in context that cannot accept a set

This is using PG v8.1.

I have a "table function" in C called "unnest".  It takes "anyarray"
as its only argument and returns a set of "anyelement".  It's a handy
little function for turning arrays into sets.

You can use it in two different ways:
    SELECT * FROM unnest(ARRAY[1,2,3]);
or
    SELECT unnest(ARRAY[1,2,3]);

The latter is particularly handy when used like this:

# select unnest(ARRAY[1,2,3]), 'hi';
  unnest | ?column?
--------+----------
       1 | hi
       2 | hi
       3 | hi
(3 rows)


I decided that this function would be easy to rewrite in PL/PGSQL and
then I could stop compiling an extra library every time I install
Postgres.

CREATE OR REPLACE FUNCTION unnest2 (_a anyarray) RETURNS SETOF
anyelement LANGUAGE plpgsql IMMUTABLE AS $$
DECLARE
    i int;
    upper int;
BEGIN
    i := 0;
    upper := array_upper(_a, 1);
    FOR i IN 1..upper LOOP
        RETURN NEXT _a[i];
    END LOOP;
    RETURN;
END;
$$;

However, the PL/PGSQL version cannot be used as a column.  Doing so
does results in an error:

# select unnest2(ARRAY[1,2,3]), 'hi';
ERROR:  set-valued function called in context that cannot accept a set
CONTEXT:  PL/pgSQL function "unnest2" line 8 at return next

Is the definition of "unnest2" wrong or is this just a limitation of
PL/PGSQL?  If this can't be done via PL/PGSQL in v8.1, what about v8.3
(or later)?

Any input will be greatly appreciated!

eric