It's also funny that on the desktop OS designed by Apple, that paragon of modern and superficially minimal design, the menu is always visible (in the menu bar, just for the focused application). Other desktops trying to be as "easy and beautiful" as OS X are doing more to hide the classic menu.
(not an apple fanboy, I promise, I've spent more time with linux overall)
Apple chose to always have an active menu at the top of the screen because of Fitts' law, which roughly says that bigger buttons can be clicked on more quickly than small ones (see wikipedia for a more precise mathematical description). Duh.
With the menu at the top, the user can slam the mouse pointer to the top edge, and simply focus on moving the pointer left of right. This is perceptually equivalent to having the screen divided into several tall stripes, one for each menu item. The menu items thus act like buttons with very large screen area.
On a small mobile device, this trick is used too: most of the UI buttons are on the edges of the display.
Compare this experience with trying to hit a tab or button on the top edge of your application's window (on a desktop computer). The top edge is rather narrow, and the targets sitting on that edge have small area. It takes much more effort to hit them, than if they were planted at the top of the screen.
From what I've read, Apple chose this design way back in the stone age era (early 1980s), after actually doing user testing!
(not an apple fanboy, I promise, I've spent more time with linux overall)