My feeling is this is a bit of social signalling going on. Woz is saying 'Gray is one of us, we can joke around about it, because that's the kind of teasing we can do in our circle'. If the matter were serious, Wozniak would indeed be out of line, and he'd hopefully know better than to do this. There's no need to keep Powell's name secret anymore, he's part of the Apple mythology now.

There is a few bits more info in this article: http://news.cnet.com/8301-13579_3-20003345-37.html

Woz:"It seems clear that (Powell) kept the iPhone prototype secret enough to satisfy Apple. It's a bad accident that could happen to any of us."