Wouldn't think theres too much wrong with that.
Its like Becks walking around in a Man Utd shirt? No one would really care too much.. Its just very short time period since he left, which makes it so bad. I don't really care as a City fan that he has done this.. Its on the pitch that you rate the players i suppose. But still, i would like an explanation. Now.
EDIT:
I just realised something else aswell, Arsenal made his career. He would have loved the club. He was best mates with a lot of the staff including Wenger and a large group of players, it made his career and his life. I lose no respect in him, sitting in his back yard, with an old shirt he got in his Arsenal days, stop over-reacting people.