For the record :
Replication formulas have no effect on cluster replication. If you create a document in a clustered database then that document will be replicated out to the other members of the cluster even if a replication formula exists on the database that would normally exclude the document from replication.
When the database is then replicated using standard scheduled replication the replication formula will kick in and any documents that should not exist in the other replicated databases will be removed.
So if your using a combination of clusters and replication formulas then remember that incorrect documents will exist in the clustered databases and that you should run a very regular scheduled replication.