Can I inter-operate the Web API with the old SOAP API?
Yes. If you are an existing user of the SOAP API, it is possible for you to start using the new framework while keeping most of your code unchanged. You simply need to replace the AppUserKey supplied to every existing SOAP call with the session key returned from the auth.getSessionKey API call. All the SOAP calls will work as before. You can also write Web API code using the same session key.