Newbie question about escaping in a function

I have a simple function defined thusly:

CREATE OR REPLACE FUNCTION datemath(timestamp with time zone, int4,
varchar)
RETURNS timestamp AS
'
DECLARE
tdat timestamp;
rdat timestamp;
BEGIN
IF ($1 IS NULL) THEN
TDAT := NOW();
ELSE
TDAT := $1;
END IF;

select tdat + interval ''$2 $3'' into rdat;
return rdat;
END;
'
LANGUAGE 'plpgsql' VOLATILE;

The problem is the interval part. How do I tell the bugger to use the
second and third params as input to interval? I have tried different
ways of escaping, from \'$2 $3\' to ''$2 $3'' and everything else in
between, it just doesn't like it.

Help! :-)

Thanks,
naeem