Hello Matt,
I believe App Center currently supports only SP initiated requests. This means that you still need to provide the username and password that is tied to your SAML IDP. In this particular case, we are passing the authentication request to your SAML IDP and using the returned token to authenticate to App Center. I believe there are ways to support IDP intiated request for the web portal, but not for the native mobile clients.
Just to get a better grasp of then environment, who is your SAML ID provider?