Merb, which was already closely patterned after Rails, brings performance, modularity, and better integration with alternative JavaScript and ORM frameworks to the table. The default Rails configuration will still be the "full stack" framework, which uses ActiveRecord and Prototype, but there will also be a "Rails Core" with the ability to opt into specific other (e.g. JavaScript or ORM) frameworks as desired.
The Merb team will be working with the Rails core team on a joint project. The plan is to merge in the things that made Merb different. This will make it possible to use Rails 3 for the same sorts of use-cases that were compelling for Merb users. Effectively, Merb 2 is Rails 3.
You can read more about here:
- http://weblog.rubyonrails.org/2008/12/23/merb-gets-merged-into-rails-3
- http://rubyonrails.org/merb
- http://yehudakatz.com/2008/12/23/rails-and-merb-merge/
- http://brainspl.at/articles/2008/12/23/merb-is-rails
- http://merbist.com/2008/12/23/rails-and-merb-merge/
- http://onrails.org/articles/2008/12/24/the-future-of-rails-rails-3-0