The problem is caused by jQuery version conflicts with mockjax. With the help from marcoow I found this stackoverflow question. With jQuery < 1.10 it works. Mhh... that's not nice.
By the way a working jsBin: http://jsbin.com/ASaSaRiX/11
EDIT: You can find more detailed information here. The problem is caused by a changed in jQuery.
@marcoow: One fix is to add dataType: 'json'
to the request options of Ember.SimpleAuth. Maybe you have some time to look at the informations given in the link above.