VACUUM on database with open transaction returns error?

In one window connect with psql to the database 'test' and execute

test=# begin transaction;

In another window as the postgrs user do

bash-2.04$ vacuumdb -z -a
Vacuuming www
VACUUM
Vacuuming template1
NOTICE: RegisterSharedInvalid: SI buffer overflow
NOTICE: InvalidateSharedInvalid: cache state reset
VACUUM
Vacuuming test
VACUUM
Vacuuming photo
VACUUM

bash-2.04$ vacuumdb -z -a
Vacuuming www
NOTICE: RegisterSharedInvalid: SI buffer overflow
NOTICE: InvalidateSharedInvalid: cache state reset
VACUUM
Vacuuming template1
VACUUM
Vacuuming test
VACUUM
Vacuuming photo
NOTICE: RegisterSharedInvalid: SI buffer overflow
NOTICE: InvalidateSharedInvalid: cache state reset
VACUUM

bash-2.04$ vacuumdb -z -a
Vacuuming www
VACUUM
Vacuuming template1
VACUUM
Vacuuming test
VACUUM
Vacuuming photo
NOTICE: RegisterSharedInvalid: SI buffer overflow
NOTICE: InvalidateSharedInvalid: cache state reset
VACUUM

bash-2.04$

Back in the first window do:

test=# COMMIT ;
NOTICE: InvalidateSharedInvalid: cache state reset
COMMIT

OK, what gives?

1. Should I worry about these messages at all?

2. Is it supported to VACUUM a database with open transactions?
If not: what am I suppoed to do?

3. Why does the NOTICE move around like this?

4. Are there any problems with my transaction in the first window?

Now I'm worried.....

--- Allan.

PostgreSQL 7.1.2-4PGDG on Linux 2.4.7 i686 SMP (glibc 2.2.2-10)