Question posted in 
Is it possible to test my app in the iPhone Simulator w/ the 3.0 SDK. The only options that I currently have available are 3.2 (iPad) & 4.0 (iphone). I know I could buy a iPod Touch w/ 3.0 off eBay or something but I’d really like to avoid spending money if it’s possible to get this working in the simulator.
2
Answers
http://developer.apple.com/iphone/library/documentation/xcode/conceptual/iphone_development/125-Using_iPhone_Simulator/iphone_simulator_application.html
Important: iOS 4.0 uses the same Objective-C runtime as Mac OS X v10.6. iOS 3.2 and earlier use the Mac OS X v10.5 Objective-C runtime. Because of this change, binaries generated with an iPhone SDK distribution earlier than 4.0 do not run in the simulator that’s part of the iPhone SDK 4.0 and later distributions. After moving from iPhone SDK 3.2 and earlier distributions to a 4.0 or later distribution, you must rebuild your iPhone Simulator binaries to run them in the simulator. If you use licensed static libraries in your application, you must obtain versions of them generated with an iPhone SDK 4.0 or later distribution. For more information about the Objective-C runtime, see Objective-C Runtime Reference.
One other option is to maintain an older copy of the SDK/Xcode on another Mac, and run the old SDK and Simulator there for regression testing. But testing on a older device is the only way to test that an app can actually perform reasonably on such older devices. Neither a new device nor the Simulator can do that.