Nas is doing an AMA at reddit http://www.reddit.com/r/Music/comments/233t3x/i_am_nas_amaa/
He was asked about ghost writing (it's the top question currently)
"How did you end up (allegedly) ghostwriting for Will Smith?
Edit: The song getting jiggy wit it."
"Alright, let's clear this up once and for all.
I hung out with Will in the studio. And watched him write it. It was a fun studio session, and I said a line or two or three to him. It wasn't that serious. Will Smith wrote that song.
But seriously, I watched him have fun making that record on his own, and Will is a true MC."