telling app to use middleware in gem

I have a gem, and part of the gem is a piece of rack middleware. In
Rails 2.3, I could make the gem set up the middleware without adding
anything to config/*.rb in the app. I did this with the following
line:

ActionController::Dispatcher.middleware.use 'MyMiddleware'

But in Rails 3, this doesn't work.

I've tried putting the following into the gem's lib/gem_name.rb:

Rails.application.config.middleware.use 'MyMiddleware'

But now when I run `rake middleware` I get "undefined method `config'
for nil:NilClass" which means that Rails.application is nil.

I successfully used the middleware by with an initializer or something
in the app's config directory, but I'd like to do this in the gem. Is
this possible?