My current project is in Rails. Coming from a Symfony (PHP) and Django (Python) background, they both have excellent admin generators. Seems like this is missing in Rails.
ActiveScaffold is available for Rails 2.3.x :) Just for someonse's info who have found this question one year later like me :)
ActiveScaffold is by far and away the most configurable/easiest to integrate/most automagic scaffolding around at the moment. It has built in ajax support, near seamless db introspection and it even plays nicely with legacy Oracle databases (which can be a real pain in Rails).
Try it: http://activescaffold.com/
Here is a roundup of a few options, including more than just ActiveScaffold.
Active Admin (http://activeadmin.info/) was released in May of 2011, and looks like it's going to become the best Rails 3 option.
ActiveScaffold is a good solution, but if you want a more configurable and powerful tool, I think Typus is a great solution: http://github.com/fesplugas/typus
Have a look at Casein (http://www.caseincms.com/), might be what you're looking for.