Visual Studio 2013 Preview was released yesterday (see also a video):
Visual Studio 2013 Preview splash
The only new documents navigation related feature I can see is Peek Definition:
Peek Definition in Visual Studio 2013 Preview
It is similar to the older Code Definition tool window, but opens inside the code editor. Only one Peek Definition sub-window can be opened at once and it is read-only.
Tabs Studio Blog (organizing Visual Studio document tabs) 