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