Using CSS for this can give you screwy and mixed results more than likely, what you want to do is use jQuery to grab the height of [font=Corbel]#leftpanel[/font] and give it to [font=Corbel]#rightmenu[/font].
CSS-wise, height: 100%; on the [font=Corbel]#maincontainer[/font], [font=Corbel]#innercontainer[/font] and [font=Corbel]#rightmenu[/font] strips with overflow: hidden; from [font=Corbel]#maincontainer[/font] removed will give you the closest result, but it's still not exact.