mystery disease -- problems creating simple table[s]

I have a small database which has been working fine. I just converted
from 6.5.3 to 7.0.2 (Debian/Linux). Used pg_dumpall to restore the contents.
What is strange now is that I can't create new tables without causing
vacuum problems. For example:

CREATE TABLE test_table (
test_id int4,
description text,
PRIMARY KEY(test_id)
);

will cause subsequent vacuums to say:

NOTICE: Index pg_attribute_relid_attnum_index: NUMBER OF INDEX' TUPLES
(564) IS NOT THE SAME AS HEAP' (573).
Recreate the index.
VACUUM.

Unfortunately the next vacuum repeats this same message. Restarting
the postmaster does no good.

'Drop table test_table' will fix things (vacumm stops complaining), but
clearly it's not very good if new tables can't be added.

Any suggestions as to how to diagnose this problem are welcome!

-frank