It's divided in three parts:
Part one: the Ruby on Rails backend and the API -http://lucatironi.github.com/tutorial/2012/10/15/ruby_rails_android_app_authentication_devise_tutorial_part_one
Part two: register and login with the Android app - http://lucatironi.github.com/tutorial/2012/10/16/ruby_rails_android_app_authentication_devise_tutorial_part_two
Part three: task creation and update - http://lucatironi.github.com/tutorial/2012/12/07/ruby_rails_android_app_authentication_devise_tutorial_part_three
I covered topics such as the registration of a new user, his login and following authentication via a token, the retrieval of objects from the backend to the Android app through the Json API and many more.
I hope you can find it useful, let me know if I can improve it.