Yes, it's a catch 22. You can't take a nandroid backup without booting a custom recovery image. You can't boot a custom recovery without unlocking the bootloader. You can't unlock the bootloader without wiping the phone, so you can't take a backup.
ICS should be out for the Nexus S officially in the next week or two, depending on how accurate Google was by saying "a couple of weeks" after the Galaxy Nexus launches. Then you wont have to wipe anything.
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Yes, it's a catch 22. You can't take a nandroid backup without booting a custom recovery image. You can't boot a custom recovery without unlocking the bootloader. You can't unlock the bootloader without wiping the phone, so you can't take a backup. ICS should be out for the Nexus S officially in the next week or two, depending on how accurate Google was by saying "a couple of weeks" after the Galaxy Nexus launches. Then you wont have to wipe anything.