immutable functions

The documentation has this to say about immutable functions...

> or otherwise use information not directly present in its argument list

If the arguments are "row variables", does this allow access to the
data in the row? For example, is it safe to make the following
function definition immutable.

CREATE OR REPLACE FUNCTION distance(geocodes, geocodes)
RETURNS double precision AS
$BODY$
select case $1.zip = $2.zip
when true then 0
else ((acos(sin(($1.lat) * (pi()/180)) *
sin(($2.lat)*(pi()/180)) + cos(($1.lat)*(pi()/180)) *
cos(($2.lat)*(pi()/180)) * cos(($1.lon - $2.lon) *
(pi()/180))))*(180/pi())* 60 * 1.1515)
end;
$BODY$
LANGUAGE sql immutable
COST 100;

Cheers,
Andy