Originally Posted by bwcorvus
Well when we rooted the xoom, even after unlocking it we couldnt do anything with the stock boot.img. We had to fastboot a rootboot.img. After this we could adb and superuser. I would assume you have to keep that rootboot.img but i have never tested it after.
My understanding is as follows: the root boot.img just has the default.prop ro.secure variable set to insecure mode, which allows you to push the binaries and do the "extra" adb commands (eg., remount). Flashing the stock boot.img puts it back to secure mode, but the binaries should still be on /system, so you won't be able to do the "extra" adb commands nor push files on secured partitions, but the superuser binaries should still be there.
(1: Of course, once Moto releases an update that removes the superuser binaries, you need to repeat the rooting steps, but that's a given)
(2: Now that we have stock boot images for the MZ600 and 604, we don't even need to use a custom kernel, we just have to unpack and repack the stock boot images)