IPhone – Finding iPhone 3G firmware version after full reset

There is an insanely detailed step by step process on how to do this using the cydia app appbackup on http://applemonday.com/2011/04/jailbreak-data-apps-ios-appbackup/ which basically grabs all your app data and puts it in one folder for you to ssh into and get.

Recent versions of redsn0w auto-detect the version of iOS running on the connected device, so you can usually just jailbreak it without having to manually figure out the iOS version first. You can always get the latest version of redsn0w from the Dev Team Blog: http://blog.iphone-dev.org/tagged/redsn0w

Jailbreaking with redsn0w also "hacktivates" (unofficially activates) unactivated iPhones (although not the iPhone 4S yet), which is convenient for these kinds of situations. Some things don't work on hacktivated iPhones (push notifications, iMessage, FaceTime), and the battery may drain more than usual, but it's fine for a temporary solution when no official SIM cards are available.