There are a few things that need to be taken care of before you can get this to work. The first thing (although, any of the following steps can be done in any order) to take care of involves your User model. You need to override the to_param method, so that Rails will appropriately use the username attribute rather than user_id when constructing paths.

  
#in app/models/user.rb
def to_param
  "#{self.username}"
end

Next we move onto routing our resources. Here it gets a little tricky because Rails is building paths for us. Remeber, you can get a list of all currently defined routes in your application by running the routes rake task

rake routes

We need to set the path_prefix option on any of our resources we want scoped by the username. For instance, in this example, I have set up a Status model and statuses_controller, whose urls shall be scoped by the username. You can apply the path_prefix to any number of other resources in your routes config file. They symbol used is arbitrary, but will be made available in the params hash, in this case params[:user_id]. You also need to exclude the show action on your users resources declaration. The reason is that, otherwise, Rails will include the controller name in the path like /users/username, which doesn't look as clean as just /username. You then need to redefine this route explicitly (last line in the routes config shown here).

#in app/config/routes.rb
map.resources :statuses,   :path_prefix => '/:user_id'
map.resources :users,     :except => [:show]
map.user '/:username' :controller => 'users', :action => 'show'

Finally, you get to call these routes in your views or use them in your controllers. You use the same link_to, url_for methods to generate paths. When constructing the resources you have setup with the path_prefix declartion, remember you need the user model as the first argument, followed by said resource.

<%= link_to(status.title, status_path(status.user,status) %>
# or in a controller
redirect_to status_path( status.user, status )

That's pretty much it. If anyone has another way of doing this let me know!

Just finishing up brewing up some fresh ground comments...