Well... now my session[:variable] works... I still don't understand why it does, or why sometimes it treats session as an array versus a hash.
Now, Rails has a fantastic suite of documentation. You can see the entire API, with examples, at api.rubyonrails.com. Not only can you find all the files, the classes, and the methods that are included in Rails, you can also click thru and see the associated source code. GREAT!
However, I have yet to find a comprehensive overview of the session manager: what files it uses, what the options are, how it works, ...
Hopefully someday this will become clear.