It's good-natured ribbing. Besides, Woz doesn't have it in him to be mean, I reckon.
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."
If you find this joke to be insensitive I'm having a hard time imagining what sort of humor you approve of, given practically every joke out there makes fun of someone/something.
I'd be willing to bet this wasn't his idea, that someone handed him the shirt and he went along with the gag as much to be agreeable as because he thought it was funny. So as far as gradations of jerkdom are concerned second prize ought to go not to Woz but to whomever produced the shirt, whose purpose requires no speculation:
http://www.zazzle.com/the_unofficial_gray_powell_iphone_t_sh...
(Edit: Deleted the sentence "It does seem like poor judgment, though, also not out of character for Woz." That was sloppily and hastily written, didn't convey what I meant, and isn't even something I believe.)
Those are the types of jokes Americans like. I prefer surreal/absurdist humor. http://www.laughlab.co.uk/