pg_dumpall blocking other sessions

Hi All,

I had a situation where a session was getting blocked while per
performing a pg_dumpall.

I could see the pg_dumpall session executing a COPY command on a table
in one schema.
Another user session was attempting to truncate a different table in a
different schema, but in the same database. This session was being
blocked by the pg_dumpall session.

I intend to test this further, but my understanding was that pg_dump
was a non-blocking operation. I'm still on the learning curve with
Postgres 8.4 (actually EnterpriseDB Advanced Server), so maybe I just
need more detail on how pg_dumpall works.
Is the truncate perhaps the problem, but other dml is allowed?

Thanks much,
Terry