As people have mentioned, rails doesn't have it built in. I've used gems to provide it since I don't trust myself to write good enough security algorithms (and really, why reinvent the wheel if I don't have to).
In .net we can use the asp.net membership. But you've always got to have that authorization part, which I think can get forgotten about unless you've got a system under you belt or something/someone to crib from.
Sometimes you just don't think, and sometimes it becomes very public.
You can even create your own custom filters.
http://www.youtube.com/watch?v=BsxUsyMSGeA
Just letting you know. :)
However, I do think that authentication is where people may believe they can stop, forgetting or maybe not understanding, that authentication really doesn't do much, without an authorization system.
How this happened is what I want to know too.