For those unfamiliar, having an insecure boot image allows you to use adb remount and adb push system files. While most custom ROMs ship with insecure boot, stock ROMs obviously do not. So if you updated your Nexus S to the stock Jelly Bean OTA, you likely have noticed that adb remount no longer works and you are now unable to adb push and adb pull files from the /system partition.
Coming from a custom ROM, XDA News Writer HQRaja (that would be yours truly) found this crippling after the JB update, so I pulled the boot image, unpacked it, set it to insecure, and repacked it. You can grab the insecure boot image the original thread or my blog and flash it to your phone using fastboot._________