apparent wraparound

I see other posts on this log message before, but not clear to me how/if
they apply to me. Opinions appreciated. I am on 8.1.3 on Linux. I have a
log entry like this:

2006-11-08 12:38:34 PST [3739]: [3-1] LOG: could not truncate directory
"pg_multixact/members": apparent wraparound

Nothing troubling in the log before or after that. No other entries in
the log for [3739]. The system is on autovacuum which seems to have run
a little before and a little after this entry:

2006-11-08 12:37:52 PST [18388]: [1-1] LOG: autovacuum: processing
database...
...
2006-11-08 12:38:55 PST [18406]: [1-1] LOG: autovacuum: processing
database...

The pg_multixact/members directory has files that are progressively
newer:

prddb0:postgres:~ ll data/pg_multixact/members/
-rw------- 1 postgres postgres 32768 Nov 10 15:08 0010
prddb0:postgres:~ ll data/pg_multixact/members/
-rw------- 1 postgres postgres 32768 Nov 10 15:13 0010
...

Anything I should be concerned about? What does this message mean?

George