D6 + Services 3 + Appcelerator = Login help!
make your endpoint 'api'

THen your login url is, 'api/user/login' POST

And also check to make sure you have allowed application/xml as a value response format at admin/structure/services/list/api/server

Although you might need to wait for this patch to be committed 
http://drupal.org/node/1232984