Jeff Atwood on the problem of the iPhone of only having one hardware button. He argues that the home button has too many things assigned to it. Think about it. You use it to get out of an app, to go to the first page, to go to the search window (if you’re in already in the first page), to put apps in wiggle mode, and many others. He put up a diagram of all the complexity that one hardware button does.
I’ve gotten to the point where I dread using the home button on my iPhone because it Makes Me Think. And I get it wrong a significant percentage of the time. This isn’t the way it’s supposed to be.