The dump begins with setting `TRANSACTION ISOLATION LEVEL SERIALIZABLE`

Please read the manual on [Transaction Isolation][1]
and for example the 
[User List discussion about this][2]

Qoute: 

> The backup will only see transactions that were commited before the isolation level was set.

Basically everything after pg_dump sets the transaction isolation level is **_not_** part of this dump.

The read/write operations are not affected(locked) during that time.



  [1]: http://www.postgresql.org/docs/8.4/static/transaction-iso.html "Transaction isolation"
  [2]: http://postgresql.nabble.com/What-happens-to-transactions-durring-a-pg-dump-td2079158.html