This sounds like a potentially very serious PostgreSQL bug. As mentioned in the earlier thread, there's no reason to suspect that either OpenACS or AOLserver is responsible. If either had DROP'd the tables, your site would've died on the spot (actually typically any attempt to DROP a table would fail due to references from other tables view foreign keys).
And of course there's not any code in OpenACS that says "DROP all OpenACS tables".
What version are you running? Do you have PG server logs stored somewhere that cover the period of time in which the failure happened?
This discussion really belongs on one of the PG group's mailing lists.
