Hey there, I was searching for the very same thing and found it in this thread.
http://wordpress.org/support/topic/155935
Thanks to recordinghacks! One thing that isn’t mentioned, the original wp_rel_nofollow() function is found in formatting.php for me I found it between lines 1205 and 1220. Copied that into my functions.php, changed the function name just like recordinghacks did.