As of Xcode 3.2.3 you aren’t allowed to use old base SDKs anymore, even if they are in the correct folder they aren’t shown as options in project settings. Use base SDK 4.0 and in the target set the deployment target to 3.0 if you need to target old devices. (http://www.clarkcox.com/blog/2009/06/23/sdks-and-deployment-targets/)