Please HELP - URGENT - transaction wraparound error

Oh my god!....

DB is pg 7.4.6 on linux

2005-10-27 05:55:55 WARNING: some databases have not been vacuumed in
2129225822 transactions
HINT: Better vacuum them within 18257825 transactions, or you may have
a wraparound failure.

2005-10-28 05:56:58 WARNING: some databases have not been vacuumed in
over 2 billion transactions
DETAIL: You may have already suffered transaction-wraparound data loss.

We have cronscripts that perform FULL vacuums

# vacuum template1 every sunday
35 2 * * 7 /usr/local/pgsql/bin/vacuumdb --analyze --verbose template1

# vacuum live DB every day
35 5 * * * /usr/local/bin/psql -c "vacuum verbose analyze" -d bp_live -U
postgres --output /home/postgres/cronscripts/live/vacuumfull.log

Questions:

1) Why do have we data corruption? I thought we were doing everything we
needed to stop any wraparound... Are the pg docs inadequate, or did I
misunderstand what needed to be done?

2) What can I do to recover the data?

I have full daily backups from midnight each day using
/usr/local/pgsql/bin/pg_dump $DATABASE > $BACKUPFILE

plus I have this database replicated using Slon 1.1.0 to another 7.4.6
database.

I can failover to the slave server, but what do I need to do to rebuild
the original database?

Should I failover now?!! And then start rebuilding the old master
database (using slon, I presume)?

How do I stop this EVER happening again??!!!

Thanks for help

John