Alright, I think I have this figured out. The issue is that I haven't provided provided admin_username and admin_password in the config (because I didn't have one to use).

If you provide the appropriate admin credentials, it will bind to AD immediately upon connect using those credentials, and Auth::attempt(...) works fine because you're already bound to AD by the time it tries to run retrieveByCredentials(...), so it can run the user query ($this->ad->user()->infoCollection($user, array('*'));) just fine. Without admin credentials, however, you're not bound yet, infoCollection(...) returns false, and you get nothing.

I entered my own (non-admin) user and pass into the appropriate config entries, and sure enough, it worked perfectly, at least when trying to Auth:attempt(...) my own account again. The next thing to try will be to get a real admin account from the network guy and see if it still works.

So the solution is basically that admin_username and admin_password are actually required, despite what the readme says, if you want to be able to use it as a direct replacement for Eloquent's Auth::attempt(...) method.