I'm interested to know how the people targeting iOS releases other than 4.3, 4.2, and 3.1 actually test on those platforms before release (ignoring the simulator for now — many apps don't work there at all, and it often has subtly different behavior that can cause crashes when running on a device).
The issue here is that Apple specifically blocks restoring to any firmware except the very latest available. Since the iPhone 3GS and iOS 3.0, only the most recent firmware released for that device is allowed to be installed onto the device. And, to be very clear: that doesn't mean no downgrades, it means only the very latest is allowed. And that's even through Xcode's Organizer (which will "helpfully" allow you to try and restore older versions, which will then return an error partially through the restore), as well as iTunes when specifying a custom file to restore with.
Because of that restriction, I'm not sure how any other version can be supported: you have iOS 3.1 on an original iPhone, iOS 4.2 that an iPhone 3G can restore to, and iOS 4.3 on the current models. But other iOS versions don't have devices that can be restored to them. Do you just test in the simulator? Try and keep a device with the older version around, and hope it never needs to be replaced or restored if something goes wrong? I'm just not sure how something like that is managed.
(Note: there is a way around the above restriction, namely, saving Apple's "okay, you can restore to this version" response (often called a SHSH hash) when Apple is currently allowing that device to be restored to a certain version, and then spoofing that same reply later when you want to restore. That's how TinyUmbrella[1] and Cydia's TSS service[2] work, but it's (of course) not supported and won't even be possible as of iOS 5.)
[1]: http://thefirmwareumbrella.blogspot.com/
[2]: http://www.saurik.com/id/12
The issue here is that Apple specifically blocks restoring to any firmware except the very latest available. Since the iPhone 3GS and iOS 3.0, only the most recent firmware released for that device is allowed to be installed onto the device. And, to be very clear: that doesn't mean no downgrades, it means only the very latest is allowed. And that's even through Xcode's Organizer (which will "helpfully" allow you to try and restore older versions, which will then return an error partially through the restore), as well as iTunes when specifying a custom file to restore with.
Because of that restriction, I'm not sure how any other version can be supported: you have iOS 3.1 on an original iPhone, iOS 4.2 that an iPhone 3G can restore to, and iOS 4.3 on the current models. But other iOS versions don't have devices that can be restored to them. Do you just test in the simulator? Try and keep a device with the older version around, and hope it never needs to be replaced or restored if something goes wrong? I'm just not sure how something like that is managed.
(Note: there is a way around the above restriction, namely, saving Apple's "okay, you can restore to this version" response (often called a SHSH hash) when Apple is currently allowing that device to be restored to a certain version, and then spoofing that same reply later when you want to restore. That's how TinyUmbrella[1] and Cydia's TSS service[2] work, but it's (of course) not supported and won't even be possible as of iOS 5.)
[1]: http://thefirmwareumbrella.blogspot.com/ [2]: http://www.saurik.com/id/12