How to unpack an unencrypted .ab android backup file created with "adb backup": drop the first 24 bytes, deflate the rest with zlib, feed to tar. (Or prepend a gzip header and feed to tar with -z)
$ dd if=my_backup.ab bs=24 skip=1 | zlib-flate -uncompress | tar xv
$ {
printf "\x1f\x8b\x08\x00\x00\x00\x00\x00"
tail -c+25 my_backup.ab
} | tar xz
(Result: "Oh. My photos aren't in this. I guess I have to backup files separately. Good thing I didn't wipe the phone yet...")