Scusi and mmm286 - you need to unlock your bootloader before you can root etc. This normally wipes your SD card. Can't you copy your userdata manually instead of using ADB backup? Also, I think I heard of a method to unlock bootloader without wiping, but don't remember it well, try searching.
Yes, I just tried this as well. It appears Google screwed up the bootloader image in the recent firmware upgrades. Download this bootloader instead:
And replace it with the one in Google package, then it all works!
Didn't work for me...tried what you said but still getting same error as above...Does Mark read any of this? There are some other bugs in the new unified toolkit for Nexus 7...where do we report defects?