The only way you can be sure that your application will work on a device running 3.1 is to test it on a device running 3.1. The same goes for support of older (pre-iPhone 3G S) hardware. I've seen far too many applications on the store that were clearly tested only on a newer device and which bog down or run out of memory on older hardware. If you don't support that hardware, great, just set your Info.plist appropriately to not make it available to those devices.
No matter how thoroughly you think you've checked for newer methods and classes, you can still run into subtle issues. For example, you may or may not need to weak link certain frameworks or your application will crash on older OSs, even if you don't use any new APIs.
Older model iPod touches are cheap on eBay and can be downgraded to the older OS versions if 4.0 has been installed on them. Alternatively, you can find someone with a device running an older OS version and send them an ad hoc build of the application to test. This is less desirable, because you'll want to hook up Instruments to the device to make sure memory usage is not getting too close to the danger area on the older device.
I'm sorry, but that's the only way to be sure. As another developer said to me, "If you haven't tested it on a specific OS version or device, consider your application to not run there."