I just installed the iPhone 2.1 OS on my 3G device, but am still using the 2.0 SDK. I can no longer build and run my app through XCode - "failed to start remote debugserver on . . . ". So I'm thinking that I will probably have to upgrade my SDK to 2.1. According to the iPhone DevCenter, the 2.1 SDK requires that any devices for testing must be on the 2.1 OS. My questions are: 1. Does this mean that any apps built using the 2.1 SDK can only be run on 2.1 devices once they are posted to the AppStore? Or is there any kind of backwards compatibility? 2. There used to be a notice saying that apps built using the beta 2.1 SDKs could not be posted to the AppStore. I don't see that notice any more, but has anyone seen any positive indications that 2.1 builds can be posted?