I released a beta version of the Ext.Direct router for Rails yesterday.
http://rubyforge.org/projects/rails-extjs/

See README.rdoc where I wrote a 3-step setup process.
Code:
>sudo gem install rails-extjs-direct
This gem is very new and I have yet to handle file-uploads from multipart forms but that won't take more than 2 hours. The Rails router is implemented using Rack Middle Ware so you need Rails 2.3.2+. The Rails gem will route transactions to multiple ApplicationControllers during a single Ajax request.

The Merb gem has been around a little longer.
http://rubyforge.org/projects/merb-extjs/

Code:
>sudo gem install merb-extjs-direct
The Merb gem is implemented with merb-parts so Ext.Direct requests will be routed to actions within your Merb::PartControllers. I'm looking forward to something like merb-parts in Rails3.

Both the Rails and Merb gem have a very small code footprint, there's really not much to them. If anyone wishes to contribute to either of these projects, let me know and I'll provide access to the repos on Rubyforge.

I foresee adding more gems to these namespaces "merb-extjs" and "rails-extjs" to assist with Rails/Merb development with Ext JS.

Please let me know of any issues or suggestions.