why do you want extra hardware buttons? learn to use one button to maximum capacity. much more elegant.
it would not be "hard" to add buttons, but a step in the wrong direction for the design which has become iconic, and which makes all optional shortcuts easy to remember and execute.
if users find themselves having difficulties with Home button combos, maybe next we'll see users complaining about complicated multitouch / gestures? indeed we might as well start demanding a simpler touch screen interface, not allowing more than one finger on the screen at a time, and instead an array of buttons around the screen. of course a smaller screen would be good, since the buttons take up space, we could demand a lower resolution screen since the present screen shows too much detail and causes headaches; black and white would be good, the colours can be confusing when looking at the home screen. some people seem to find the united inbox approach of OS4 too difficult to handle --> we could ask apple to limit the number of mail accounts which can be accessed from an iphone. it's so complicated to remember which account you're checking. instead carry multiple phones , each for one mail account- and please: no forward facing camera, we'll never work out which one is which,
here's the device i would propose for you. guaranteed without key-combos. i'm sure you agree it's the obvious direction future iphone hardware should follow - very elegant and non-confusing