semi-RESTful routes user problem

I am extending my application to provide output in either html or xml
for most actions. Following the guidance of the default routes.rb I
modified my index route from:

  map.index '/index/:action',
              :controller => 'index',
              :action => 'show'

which works fine to

  map.index '/index/:action.:format',
              :controller => 'index',
              :action => 'show',
              :format => nil

which fails with "No action responded to index" (no changes to
controller code, and its the first route in routes.rb). I guess that
the routes architecture is changing quite a bit, but i am not on edge
rails, I'm on (I think the latest gem version).

What is the right rails approach to route construction for this? And
where have i messed up on the above?

Cheers, and thanks, --Kip