Using gems within Engines - need to 'require'?

I'm building an engine and as part of that I want to use acts_as_list.
So I've added 's.add_dependency "acts_as_list"' to my gemspec file. When
I bundle acts_as_list gets installed as I would expect, and is listed by
'gem list'

However, when I go to use acts_as_list in my model I get method not
found errors ("`method_missing': undefined method `acts_as_list'").

To get it to work I had to add "require 'acts_as_list'" to the start of
my model file. The model is in app/models and not in
test/dummy/app/models.

This isn't too much of a fag to do, but I worry that it means I haven't
configured something correctly. Is this normal when working with
Engines?

Rails 3.2.8, Ruby 1.9.3, Ubuntu 12.04.1 LTS