Changing the id parameter in Rails routing

Using Ruby on Rails 3's new routing system, is it possible to change the default :id parameter

resources :users, :key => :username

come out with the following routes

/users/new
/users/:username
/users/:username/edit
...etc

I'm asking because although the above example is simple, it would be really helpful to do in a current project I'm working on.

Is it possible to change this parameter, and if not, is there a particular reason as to why not?