We have several iOS devices, and all of them are jailbroken. Apple put restrictions on what versions of their software you can run on the hardware you purchased from them. They literally say today you can install version X, but tomorrow you can only install version Y. When you try to install the operating system on your iPhone/iPod/iPad, they make you ask them (over the internet) if it's ok! I think that the walled garden they are trying to create started feeling like a prison long ago.
The technology they use to keep the door of the prison shut is based on digital signatures. You have to get an apple 'signature of permission' to install a particular version of the operating system that runs on the device. This signature is specific to both that exact device and software version combination. You can and should save these digital 'signatures of permission' so that you can install older software after apple decides to stop granting permission.
I recently had to do that myself using a combination of Firmware Umbrella and the fact that Saurik's Cydia retrieves and stores these signatures for you if you ask it to. If Saurik has your specific device + software version signature, then you can ask him for it when you try to install new software on your phone.
Basically you change your computer to go to Saurik's server at 74.208.10.249 when you ask to goto gs.apple.com. It pretends to be that apple server that hands out signatures (or that refuses to once they choose to force you to upgrade). I was stuck for a while until I realized that you need to put the device in DFU mode.
Even if you don't want to jailbreak for whatever reason, I would suggest saving your 'signatures of permission' (SHSH blobs) with Firmware Ubrella.
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