How can I get a base SDK that supports 3.x iPhone apps w/ the latest build of Xcode?

I'm about to push my app 开发者_StackOverflowto the app store and wanted to make sure that the base SDK is 3.x friendly. I know that if my app is tested using the 4.2 base SDK (deployment 3.1.3) but when do the final distribution build for the app store won't the app "require" 4.2 if that is my base SDK?

If it is - how can I get an app to the app store that works on both 3.x and 4.x phones?

Thank you in advance

No, if you set your deployment target to 3.1.3, then you can run on 3.1.3. You should however, always test your builds before you submit on your target iOS versions. Never just assume it'll work because you've set that field.

Also remember that because your base sdk is set to 4.2, and that will give you access to 4.0, 4.1, etc APIs, you should always ensure you're only using APIs common to your lowest version you want to be supported, in this case 3.1.3. The SIM and your 4.x devices may not give you any indication you're using an API that's not supported on 3.1.3.