pg_dump makes our system unusable - any way to pg_dump in the middle of the day? (postgres 8.4.4)

I read in the pg_dump man page that pg_dump does not block other users
accessing the database (readers or writers).

In practice, if I pg_dump our 100 GB database, our application, which
is half Web front end and half OLTP, at a certain point, slows to a
crawl and the Web interface becomes unresponsive.  I start getting
check_postgres complaints about number of locks and query lengths.  I
see locks around for over 5 minutes.

I've had to abort pg_dump as it made the system unusable.

Can I pg_dump without breaking the system?

Best,
-at