just to add to this, if you use the latest patch versions it makes setting the show/hide of the left/right sidebars more obvious when arranging panels
Steven, I do realize that both left and right sidebars show on hover, AND athat left and right main panels are (permanently) stuck.
Venthe, have a look at the configuration panel
the left and right sidebars don't have to be permanently set to auto-open (show on hover).
they can also be permanently closed or open
you can even have little icons in the status bar to manually open/close the left/right sidebars
You can have the navigator in the left sidebar permanently on display if you want too, just like your MB2.5 set-up