Weird.. I created an example scaffold here just to test and it worked fine =(. But I tested it within SimpleForm labels i18n, not AR models.
I've also created another app, with a new scaffold and change the i18n file like this:
en:
hello: "Hello world"
activerecord:
models:
admin/user: "Usuário"
And the console gives me the right values:
>> Admin::User.model_name.i18n_key
=> :"admin/user"
>> Admin::User.model_name.human
=> "Usuário"
I'm not sure the problem would be your configuration. I used Rails 3.1 as well in the examples, but I'm using REE (which I think should not be the problem).
Could you try creating a bare app to see if that works? Maybe also try using SimpleForm labels scope, as I've mentioned, just to check? If that still does not work for you, I'll have to ask you this example app showing the issue so we can check. Thanks.