Reliably finding temporary table

We have some functions which need to dynamically create a temporary
table if it does not already exist. We use the following function:

CREATE OR REPLACE FUNCTION test_date_time_exists() RETURNS BOOLEAN AS '
BEGIN
RETURN EXISTS (
SELECT * FROM pg_class
WHERE relname = ''test_date_time''
AND pg_table_is_visible(oid)
);
END;
' LANGUAGE 'plpgsql' STABLE

However, this has been failing occassionally with errors like

ERROR: cache lookup failed for relation 3454264547

From looking in the list archives, I found a description of
pg_table_is_visible failing because it has different snapshot
semantics than the SELECT. Is there a solution for this problem? Is
there another function I can use? What is a better way to detect temp
tables visible to the session?

- Ian