"app is not supported on this device" ios 4.2.1

I have given the deployment target of the app as 4.0.It s in app store.Bu开发者_开发技巧t I am having an issue downloading the app. I get the message saying that the "app is not supported on this device". I am using an iPhone 3G to try and download, using iOS 4.2.1. Do you know why this is happening?

There are a number of reasons why this will happen. See my answer to Is it possible to target older iOS versions when using Xcode 4.2 and iOS 5 SDK?

There may be other reasons apart from the OS version. Have you specified any values for the 'Required Device Capabilities' key in your plist file? It is possible that any values there are not met by iPhone 3G (such as armv7).