Every once in a while you have to compile an older project with an iPhone SDK you may no longer have installed. Apple removes links to the older versions even though the SDKs are still on their servers.
Please note that you need to be logged into your iPhone Developer Account
- iPhone SDK 3.1 with Xcode 3.1.4 Leopard (10.5.7)
- iPhone SDK 3.1 with XCode 3.2.1 for Snow Leopard (10.6)
- iPhone SDK 3.1.3 with XCode 3.1.4 for Leopard (10.5.7)
- iPhone SDK 3.1.3 with XCode 3.2.1 for Snow Leopard (10.6)
- iPhone SDK 3.2 with Xcode 3.2.2 for Snow Leopard (10.6)
- iPhone SDK 4.0 with Xcode 3.2.3 for Snow Leopard (10.6.2)
Update: Removed the older links that are no longer hosted on adcdownload.apple.com.
Update 2: Changed links from developer.apple.com/iphone to developer.apple.com/ios