It's a design standard: place your important and/or often-used design elements in the upper right corner of your design. Take a look at the home page of some of your favorite sites. You'll notice convenient links to maps, contact us pages, search boxes, login/logout fields, and more -- and most of these elements are in the upper right corner of the page.
It's become so familiar that most of us know instantly that when we want to search or login, our eyes immediately check the upper-right hand corner first. Instead of going into the psychology of why that is, here's the reason I think a change is coming.
On the iPhone, your navigation is done through a touch-based system. To activate the home-screen, you hit a conveniently-placed home button on the bottom-center of the phone.
It's round and concave--perfectly (and intentionally) suited for thumbs, big and small. That being said, most people cradle their phone such that their thumb is always hovering near the home button. The iPhone is 4.5" tall. If you have a thumb that's long, congratulations on your pro basketball career. But if you have an average-size thumb like the rest of us, you'll have to move your thumb from the bottom of the screen to the top, or use a finger to do the same.
My thumb's range can comfortably cover the bottom 2 or 3 out of 5 rows. So, for convenience and speed, I've reorganized my icons so that the most-used ones are near the bottom of the iPhone's screen. Apple does this too, with "Phone," "Mail," "Safari," and "iPod" organized by default as the bottom-row on all their iPhones.
This is quite a bit different design pattern than what we are traditionally used to: placing often-used elements at the bottom of our designs. As touch-screens rapidly appear on our phones, and will eventually make their way to our laptops and desktops, we will see different design paradigms that shift the focus of icons, elements, input fields, and more to the bottom of our screens, rather than the top.