Vacuum Complains

Hello;
when running a daily vacuum, my cron regularly reports this
message from Pg (7.0 RC1 / RH5.2 with glibc2.1 / compiled
with gcc 2.5.2):

NOTICE:  RegisterSharedInvalid: SI buffer overflow
NOTICE:  InvalidateSharedInvalid: cache state reset

Does anyone know what it means exactly ?
The message comes every day... no incident has been reported,
and the DB works just fine, whatever vacuum complains :)
But I would like to know if something should be done.

TIA
Fabrice

--
"Les forces de l'ordre sont celles qui sont aux ordres de ceux qui les
donnent." -- Pierre Dac