I think this is pretty obviously a fake.
I, for one, am glad of that for a couple of reasons.
First, the logic of having the tabs start at the left get added to the right is a hard one to argue with. When I was using Camino, before I switched to Safari, I thought this was a silly argument. But now, having worked with Safari for a while I really wouldn't want to go back. (The one thing that's been missing in all implementations of Tabbed browsing that I've used, including Safari's, is the ability to rearrange the order of the tabs. Anyone know if that might be available in v1.1?)
Second, as has already been noted several times, this screen shot has eliminated the close X on the tabs. That's a killer right there. And, as to the argument that it doesn't matter because there will be a new two button mouse, I'd say that it still matters, as it's the difference between click and click-drag-click, which is enough to get very annoying.
Finally, there is the issue of wasted screen realestate. When viewing web pages I'm sure that Apple is aware that people want to make the most out of their screen space. The wasted space around the edges of the window in this mock-up is just that: wasted space.
Simply, it's fake.