Jeff...essentially Tiger has no contract with Nike. At least not of a nature that prevents him from playing something else. He said on TV in a commercial that his contract says if he finds something he likes better than the Nike club he can play them.
Also, those clubs might have the Nike logo on them but you won't find any Nike club available to the public off the rack that is like those clubs. He meets with them and they specifically build and design each club just the way he wants it. From what you can see of the clubs in the glimpses you occasionally get on TV, I think that the irons he's playing now that say Nike on them look an awful lot like the Titelist irons he used to play. It's like he had Nike design his new irons based on the way his old Titelist's looked. Titelist made one of the nicest forged irons of all time IMO. back in the day many pros played them.
I have always been a forged club player. The new game improvement clubs are definitely easy to hit but to me I don't like the sound, ot the feel of a cast club. There is NOTHING like the sound, and the feel, of a well struck ball with a forged iron. Mizuno is my choice of weapon and I am even playing their forged woods. MAN, they feel good!!! Forging DEFINITELY makes a difference IMO anyway.
Mike