Best practices to run time offset delayed replication
I haven't looked into the built-in replication in Postgres 9, but if you're using Slony-I with log shipping, you can just wait to apply the logs files 'til after they're a given age.  We use a perl script under cron that connects to the master server and pulls the log files, then applies them.  It should be nothing to pull stuff to the replica to queue, and then go through and apply them, stopping when you hit the ones that are too new.