While looking at the screenshots what came to my mind is that this seems just so wrong. I wonder if it is not better to improve task switching in the window manager, pager and window list, and have one window per document, as would be natural in a document-centric desktop.
Why are people trying to ressurrect the MDI model, which the GNOME HIG despises?
In other news, good to see planning on GNOME 3.0. And it does look like people are aware that repeating KDE 4/GNOME 2 is not a good idea, and planning the breakage/delay to be limited, and sane.
Update: a conversation at #webkit-gtk@Freenode just made me realize that the tabs for the calculator and totem were probably just jokes; I really hope so! =)