Mac OS X doesn't care how you arrange them. Vertically is fine. If you have more than one monitor connected, when you open your Displays preference you will see icons representing however many monitors you have connected. All you do is move those little icons around until they match the way you have them arranged in the real world. That way, when you move the mouse up, it will know to pass through the top edge of the bottom monitor and enter the bottom edge of the top monitor.
Multiple monitors have worked this way on the Mac for over 20 years. 10 years ago I used a vertical arrangement when I had a PowerBook on my desk, and an external monitor plugged in which sat on a shelf above the PowerBook.
In a Mac Pro, you can keep adding more video cards and plugging in more monitors and the mouse will go between all of them as if they were one huge screen as long as you have told the Displays preference how you arranged them.
Al Gore
uses three Apple monitors. Horizontally though. His solution for desk clutter? Put it under the monitors 🙂