I was using xCode v3.2.3 and SDK 4.0 to write iPhone apps that worked on my iOS v3.1.3 device... as well as v4.0. (Deployment target set to 3.1.3. Base SDK set to v4.0)
Does that mean everyone that uses my app will need a minimum of iOS 3.1.3 and a maximum of v4.0? Or will they also run on v4.1 and v4.2?
Second part of my question:
I now upgrade my iOS v3.1.3 to v4.2.1.
Xcode now says my v4.2.1 device is now no longer provisioned.
Doesn't a Base-SDK setting of v4.0 mean apps will also run (or at least install) on v4.x?
No. Base SDK just means that your app will be built against the iOS 4.0 libraries. Any 4.x device will be able to run a release build of your app, but for debugging, Base SDK and the iOS version on your device must match. In the end, you need to build your app with the newest available Base SDK anyway, because Apple won't accept apps built for older SDKs into the App Store.