Editing contents of the root filesystem of your Android phone

The linked page describes it pretty well. However in case of ICS the /dev filesystem has changed a bit. The boot and recovery images are not available as /dev/mtd/mtd1 (etc.) anymore. You've to look for them here: /dev/block/platform/omap/omap_hsmmc.0/by-name/boot.

Thus saving the boot image requires a commandline like this:
cat /dev/block/platform/omap/omap_hsmmc.0/by-name/boot > /mnt/sdcard/boot.img