多个操作系统版本的iPhone应用程序的最佳实践

I'm developing an app that uses iOS4 features (like MFMessageComposeViewController), but I want to support iOS 3.1.x as well. I know how to do that in code and build settings, but it's getting hard to find devices to test on--I've upgraded my phone and so have most of my friends. And the simulator with XCode 3.2 doesn't support simulating 3.1 as far as I can tell.I'm developing an app that uses iOS4 features (