2.3 to 3.1 upgrade: RESTful routes now required when using link_to("foo", @object) ?

I'm upgrading my application from Rails 2.3.11 to 3.1.1. After
updating my routes file with the new syntax, I'm seeing a change in
behavior of the link_to method when passing an ActiveRecord object as
the link destination.

Here is an example of an old (non-RESTful) route from my 2.3
application's routes.rb: