As has been suggested, I use synthetic identifiers for my data entities. This is great. But let's see I'm looking to process a login. i have the username, which while not my hibernate identifier is a unique key. Is it possible to get() or load() my user object via this natural key?

You cannot use load or get. You should use a HQL query e.g. session.createQuery("from User where username=:name").setString("name", name).

You can also declare your username field as natural id. However you cannot do this using annotations (only in the next version) only with hibernate.classes.xml. In addition as I remember there are two outstanding bugs in Hibernate core related to immutable natural ids, one was fixed in the very last version.

The advantage of immutable natural ids over a simple index is that it is cached. You have to use the criteria queries api for this, see section 15.9. Queries by natural identifier in the reference for more information.