Will we still be able to use Compiz?
In a way yes: You will be able to switch away from all the fancy new stuff and just use the gnome-panels like you did with Gnome 2. In this mode it should not be too difficult to replace the WM.
However, in standard, fancy mode you will only be able to use Mutter aka Metacity 3.
Gnome 3 is just too different, it uses lots and lots of composite effects to provide the overlay, animations and a new concept of workspace.