Not really. There have been several attempts since 2005 to create Super Auto-Scaffolding (streamlined, ActiveScaffold, and several other generic names that I don't remember, like "Ajax Scaffolding"). These aren't identical to Django's admin, but they could serve a similar purpose.

For all of its innovation, I'm a little surprised that the Rails community hasn't put together a quality solution for this problem yet. I assume the ORM differences are a part of this - Django's ORM (I think?) defines the schema in the application, while ActiveRecord gets this from the database directly. But does that really make a Django-style admin impossible with ActiveRecord?

Not that I've seen (maybe in a plugin?) That being said if you use scaffolds to create your models, some of this functionality will be there (CRUD actions on models). You still would need to add a navigation wrapper around these (pretty easy) and the authentication.