Hi,
I am using jsf 2.0 and I had a similar problem with the one described here (identical actually) and I solved it by using the following code for logout:
In my case "Admin.xhtml" is your index.html file. I suspect that response.sendRedirect() does the trick, but I do not know why. Maybe a more informed person can shed light on it.
The same code, with a different signature though (i.e. did not work.
Regards,
VRoy