transaction-wraparound data loss

I run the following nightly using cron against my 7.3.3 databases:

vacuumdb -a -f -z

The other night, I got the following (db names have been changed to protect
the innocent):

Vacuuming db1
WARNING:  Some databases have not been vacuumed in over 2 billion
transactions.
You may have already suffered transaction-wraparound data loss.
VACUUM
Vacuuming db2
VACUUM
Vacuuming db3
VACUUM
Vacuuming template1
VACUUM

I run this nightly and I don't think there is any way that I could have
processed 2 billion transactions in a single day.  The first database "db1"
really isn't even used actively.

Could the error message have been generated in error?  Is there something I
should check or do?  Does this indicate that I'm going to have catastrophic
errors with one or all of my databases?

Any insights are much appreciated.

Brian E. Pangburn