Nothing major to add, except for those suggestions:
1) Document __call() with @method
2) Maybe double check the options passed into the class, if you require a url/endpoint, it wouldn't hurt to check if it's set before SOAP throws an error (imho). My point extends to 'app_credential', etc. as well.
I also like the idea of a config object which you can ask for info, Services_Atlassian_Crowd_Config::getServiceUrl(), etc.. But that might be too much. ;)
3) I think @access tags are not necessary in PHP5.
Good work! :)