I upgraded my dev setup from pg 8.2.3 to pg 8.2.4 and slony 1.2.8 to
1.2.9 today and it went smoothly, except that one slony node failed to
come up with the messages:

FATAL  localListenThread: "select "_rep1".cleanupNodelock(); insert
into "_rep1".sl_nodelock values (    1, 0, "pg_catalog".
pg_backend_pid()); " - ERROR:  duplicate key violates unique
constraint "sl_nodelock-pkey"
FATAL  Do you already have a slon running against this node?
FATAL  Or perhaps a residual idle backend connection from a dead slon?

I checked the process list and found no extra slon daemons or residual
backend connections, so then I took a look at the sl_nodelock table
for the affected node and noticed that the nl_backendpid listed for
that node happened to be the same as the pid for the currently running
pg daemon itself so the cleanupnodelock function wasn't cleaning up
the entry.

After restarting postgres the affected slon node came up normally. I
suspect I could have manually cleaned out the sl_nodelock table as
well.

I don't know enough about the _Slony_I_killBackend function, but
perhaps it could be improved to detect this situation.

-Dave