Why isn't my iPhone OS supported by XCode? It's listed right in the error message

The version of iOS on “iPhone” does not match any of the versions of iOS supported for development with开发者_JS百科 this installation of the iOS SDK. Please restore the device to a version of the OS listed below, or update to the latest version of the iOS SDK; which is available here.

OS Installed on iPhone
4.2.1 (8C148)

Xcode Supported iOS Versions
Latest
4.2.1 (8C148)
4.1
4.0
4.0.2
4.0.1
3.2
3.2.2
3.2.1
3.1
3.1.3
3.1.2
3.1.1
3.0
3.0.1

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You need Xcode 3.2.5 posted to the Apple Dev site on November 22 when 4.2.1 was posted. The Xcode 3.2.5 that came with the release of 4.2 is saying it supports 4.2.1 but is not actually doing it.

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I know its the most obvious thing to do, but have you tried restarting both your iphone and xcode?