Slony-I does not automatically replicate
Changes to large objects (BLOBS)
Changes made by DDL commands
Changes to users and roles
The main reason for these limitations is that Slony-I collects updates using triggers, and neither schema changes nor large object operations are captured by triggers
There is a capability for Slony-I to propagate notably DDL changes if you submit them as scripts via the slonik SLONIK EXECUTE SCRIPT operation. That is not handled "automatically;" you, as a database administrator, will have to construct an SQL DDL script and submit it, via SLONIK EXECUTE SCRIPT.
If you have these sorts of requirements, it may be worth exploring the use of PostgreSQL 8.0+ PITR (Point In Time Recovery), where WAL logs are replicated to remote nodes.