First of all, you need an apple developer account which can be created for free on http://developer.apple.com. Guess what, you can't download older versions of Xcode and the iOS SDK. I always knew that apple is EVIL! They try to force you into buying their latest OS release to develop iOS applications. But if you're still using Leopard (10.5.x) you can download a working version here.

source: http://iphonesdkdev.blogspot.com/2010/04/old-versions-of-iphone-sdk.html