Besides dumping the calendar app (I'm never going to be that organized) and putting the camera app there; I moved the mail app to the front right button (it was option-mail before), and in the process of using it for a couple weeks, I discovered it was designed to go there.
One day while already in the mail app, I accidentally pressed the button againand it did something useful: it switched to the next mailbox (my mail is setup with three accounts: sprint, extreme, and gmail). The alternative methods are less than ideal, especially if you don't have the stylus out, so its pretty obvious that this method was designed to be the main way to switch.
So why didn't the phone come that way? Cause its a phone, and not a handheld. SMS & MMS are more important services to the carriers (especially with how some of them charge per message), so they naturally want their apps out in front and the more generic apps (like email) behind. Heck, I'd probably even put the web in front of the phone app if I could (which I can't-darn it).