Plpgsql function with unknown number of args

I need to create my very first function. I'm using 8.0.2 and I need a
function that I can call (from my client app) with an unknown number of
criteria for a select query. The function will then return the results
of the query. In my mind, it would go something like what I've outlined
below. I realize that there are syntax mistakes etc, but this is just
an example:

CREATE TABLE mytable (
a INTEGER UNIQUE PRIMARY KEY,
b VARCHAR(100) NOT NULL,
);

CREATE FUNCTION myfunc(criteria) RETURNS ???? AS $$

BEGIN

BEGIN
FOREACH crit IN criteria
critsql := "b = 'crit' OR "
NEXT crit
END;

PERFORM "SELECT a, b FROM mytable WHERE (" critsql ");"

END;
$$ LANGUAGE plpgsql;

Select * from myfunc(1, 2, 3, 4) would then equate to SELECT a, b FROM
mytable WHERE (b = '1' OR b = '2' OR b = '3' OR b = '4');

Select * from myfunc(9, 21) would equate to SELECT a, b FROM mytable
WHERE (b = '9' OR b = '21');

My question is how do I do that? I've looked through the docs and can't
find what I'm looking for. I'm assuming this is possible because it's a
relatively simple task.

Mike