I found what I would consider to be a bug with the assert_redirect_to. Basically when the code in question performs a redirect_to_path the assert_redirect check breaks. Was puzzling me a little bit, especially with the "assert" throwing the errors.
Recreate make a piece of code that redirects via a redirect_to_path and run the test, it will throw the following stacktrace:
NoMethodError: undefined method `[]' for nil:NilClass
/usr/lib/ruby/gems/1.8/gems/actionpack-1.5.1/lib/action_controller/assertions/action_pack_assertions.rb:140:in `assert_redirected_to'
/usr/lib/ruby/gems/1.8/gems/actionpack-1.5.1/lib/action_controller/assertions/action_pack_assertions.rb:140:in `all?'
/usr/lib/ruby/gems/1.8/gems/actionpack-1.5.1/lib/action_controller/assertions/action_pack_assertions.rb:140:in `each'
/usr/lib/ruby/gems/1.8/gems/actionpack-1.5.1/lib/action_controller/assertions/action_pack_assertions.rb:140:in `all?'
/usr/lib/ruby/gems/1.8/gems/actionpack-1.5.1/lib/action_controller/assertions/action_pack_assertions.rb:140:in `assert_redirected_to'
/usr/lib/ruby/gems/1.8/gems/actionpack-1.5.1/lib/action_controller/assertions/action_pack_assertions.rb:136:in `assert_block'
/usr/lib/ruby/gems/1.8/gems/actionpack-1.5.1/lib/action_controller/assertions/action_pack_assertions.rb:136:in `assert_redirected_to'