Waiting for select

Can someone explain this?  I seem to have a query which is being blocked
by a lock.  I was under the impression that selects are never blocked.
Am I missing something or is this bad behaviour?

I am using slony and am synchronising a slave for the first time.  In
the hope of seeing some progress on the slave I attempt to perform a
select count(*) on one of the tables.

The select just stops.  ps shows this:

postgres  5987  0.0  0.4 19180 4188 ?        S    15:16   0:00 postgres: postgres testdb 192.168.1.111(33598) SELECT
waiting

A query of blocking locks shows this:

          object           | trans | pid  |        mode         | blocker
---------------------------+-------+------+---------------------+---------
 testdb.campaign_cost_pk   |       | 5754 | AccessShareLock     |
 testdb.csn_log_pk         |       | 5754 | RowExclusiveLock    |
 testdb.pg_trigger         |       | 5754 | AccessShareLock     |
 testdb.pg_trigger         |       | 5754 | RowExclusiveLock    |
 testdb.sl_subscribe       |       | 5754 | AccessShareLock     |
 testdb.csn_log_idx2       |       | 5754 | RowExclusiveLock    |
 testdb.campaign           |       | 5754 | AccessShareLock     |
 testdb.campaign           |       | 5754 | RowExclusiveLock    |
 testdb.campaign           |       | 5754 | AccessExclusiveLock |
 testdb.pg_rewrite         |       | 5754 | AccessShareLock     |
 testdb.pg_rewrite         |       | 5754 | RowExclusiveLock    |
 testdb.computer_sn_log    |       | 5754 | AccessShareLock     |
 testdb.computer_sn_log    |       | 5754 | RowExclusiveLock    |
 testdb.computer_sn_log    |       | 5754 | AccessExclusiveLock |
 testdb.address_type_pk    |       | 5754 | AccessShareLock     |
 testdb.campaign_pk        |       | 5754 | AccessShareLock     |
 testdb.sl_log_1           |       | 5754 | AccessShareLock     |
 testdb.sl_log_1           |       | 5754 | RowExclusiveLock    |
 testdb.csn_cookie_idx1    |       | 5754 | AccessShareLock     |
 testdb.pg_index           |       | 5754 | AccessShareLock     |
 testdb.pg_index           |       | 5754 | RowShareLock        |
 testdb.csn_log_idx3       |       | 5754 | RowExclusiveLock    |
 testdb.csn_cookie_pk      |       | 5754 | AccessShareLock     |
 testdb.sl_log_2           |       | 5754 | AccessShareLock     |
 testdb.sl_log_2           |       | 5754 | RowExclusiveLock    |
 testdb.sl_set             |       | 5754 | AccessShareLock     |
 testdb.sl_set             |       | 5754 | RowShareLock        |
 testdb.campaign_cost      |       | 5754 | AccessShareLock     |
 testdb.campaign_cost      |       | 5754 | RowExclusiveLock    |
 testdb.campaign_cost      |       | 5754 | AccessExclusiveLock |
 testdb.sl_table           |       | 5754 | AccessShareLock     |
 testdb.sl_table           |       | 5754 | RowShareLock        |
 testdb.sl_table           |       | 5754 | RowExclusiveLock    |
                           |  9182 | 5754 | ExclusiveLock       |
 testdb.computer_sn_cookie |       | 5754 | AccessShareLock     |
 testdb.computer_sn_cookie |       | 5754 | RowExclusiveLock    |
 testdb.computer_sn_cookie |       | 5754 | AccessExclusiveLock |
 testdb.pg_attribute       |       | 5754 | AccessShareLock     |
 testdb.sl_config_lock     |       | 5754 | AccessExclusiveLock |
 testdb.sl_trigger         |       | 5754 | AccessShareLock     |
 testdb.csn_pk             |       | 5754 | AccessShareLock     |
 testdb.pg_class           |       | 5754 | AccessShareLock     |
 testdb.pg_class           |       | 5754 | RowShareLock        |
 testdb.pg_class           |       | 5754 | RowExclusiveLock    |
 testdb.address_type       |       | 5754 | AccessShareLock     |
 testdb.address_type       |       | 5754 | RowExclusiveLock    |
 testdb.address_type       |       | 5754 | AccessExclusiveLock |
 testdb.pg_namespace       |       | 5754 | AccessShareLock     |
 testdb.pg_namespace       |       | 5754 | RowShareLock        |
 testdb.csn_log_idx1       |       | 5754 | RowExclusiveLock    |
 testdb.computer_sn        |       | 5754 | AccessShareLock     |
 testdb.computer_sn        |       | 5754 | RowExclusiveLock    |
 testdb.computer_sn        |       | 5754 | AccessExclusiveLock |
 testdb.address_type       |       | 5987 | AccessShareLock     |    5754
                           |  9422 | 5987 | ExclusiveLock       |    5754
(55 rows)

All responses will be welcomed.

__
Marc