There is no rmon-mib in RHEL 5.3 (=net-snmp-5.3.2.2-5), therefore I cannot fix a leak in it! Rmon-mib was added in RHEL 5.4 (=net-snmp-5.3.2.2-7, bug #468832).
Do I miss something? Can you please provide a reproducer? Maybe there is something else leaking file descriptors...