How to build against older iOS versions with the latest Xcode downloads?

This became harder in Xcode 3.2.5 (iOS 4.2.1) as the separate "Base SDK" values for Device and Simulator are replaced with a single "iOS SDK".

See my post in Xcode/Simulator: How to run older iOS version? for details on how to access older Simulator SDK versions in this latest Xcode.

To make an application target that runs on multiple versions of iOS is relatively simple: Set the “Base SDK” in your projects settings to the newest version number of iOS whose features you may want. Set the “iPhone OS Deployment Target” to the oldest version number of iOS that you will support

you will need to install older versions of xcode since the newer simulator will only support the newer ios versions

http://developer.apple.com/library/mac/#documentation/Xcode/Conceptual/XcodeCoexistence/Contents/Resources/en.lproj/Details/Details.html