As for the meaning of Acked-by, I'm not so sure. We currently use Signed-off-by as a tagging of "I wrote (parts of) this code" (i.e. is has legal importance), and the Acked-by merely as "ok, looks fine" indicator (but you didn't write the code).
Signed-off-by means "I am (in part) responsible for this ending up in thid repo", i.e., you wrote part of the patch or you were on the path pushing it in.
Not sure how exactly the Linux kernel folks use these...
Acked-by is used as a comment "looks fine by me" when not taking direct action yourself.
Segher