If you use Slony, expect it to lose the replication status.
I attempted the following:
1. Master and slaves on 8.4.
2. Upgrade one slave to 9.0. Shut it down, used pg_upgrade to perform
the upgrade.
3. Restarted the slave.
Slony appeared to come up, but said it was syncing only TWO tables (out
of the 33 in the working set!) Of course that didn't work very well at
all.....
Dropping the replication set and re-adding it appears to be fine, but of
course that caused a full database copy. Not too cool.
I have no idea why it only thought there were two tables in the
replication set. Very, very odd stuff....
Something to be aware of - I haven't figured out why it did this, nor do
I know if it will do the same thing to me when I attempt to upgrade the
master to 9.0 - that's something I won't attempt until the weekend at
the earliest.
Other than that running with 8.4 for the master and 9.0 for the slaves
appears to be ok.
-- Karl