Well, mine's was the right one, but it turns out my mom's laptop died the same day I got it, Good Bye Compaq FS740...Oh, btw, middle is if you have a 3-way setup (LUCKY!).

Moot question.

In dual monitor setups, it would mostly be on the left. Westerners read from left to right. Therefore, its more natural to be doing your main work on the main monitor, and having other things you might be reading as well on your right, so when you turn your head right, you'll reach the left side of the article (or chat window, etc), where you start reading.

Top bottom is also a viable setup, however, for dual monitor setups, this would require either a specially created desk, or an after-market stand, both of which will not be economically viable.

Not to mention it doesn't quite make sense as well, other than the fact we read from top to bottom as well (all languages do that AFAIK.)

Also, if you have more than one monitor, it would be common sense to position it as close as possible to the middle of the whole setup. Again, this is due to having to turn your head as little as possible. This is true for 3, 4, or even 8 monitors (yes, I've seen 8 monitor setups in some programmer's workstation).