Hardware buttons on mobile devices: Which ones should be standard?

Android phones generally have at least volume-rocker, home, back,  search and power buttons. Windows Phone 7 devices must have these plus a camera button (but home is called 'start'), with strict rules that software cannot override the basic function of these buttons.

iOS devices, have a nice 'mute' switch, but lack the hardware 'back', 'search' and 'camera' buttons. I think a strong argument should be made to add at least one, if not two additional hardware buttons to improve user experience.

Two situations I think justify a hardware button:

1. An action that should be common to all apps. 'Go back' qualifies under this criteria. This ought to go back to the previous 'flagged' state of the current app (e.g. a previous web page), or to the previous app if there has been no flagged state since arriving at this app. Smooth transition forward and back among apps would be much easier. The 'go back' should be unlimited, to the greatest extent possible, so you could go back to where you were 5 or 6 actions ago.
2. An action that may need to be performed urgently, or with gloved hands, ideally without looking at the screen too much. Taking a picture, displaying one's current location on a map and phoning a key number from one's list of favourites qualify for this, in my opinion. 

If you apply both the above criteria, it would suggest four additional hardware buttons (back, phone, camera and map) in iOS. This is likely too much for Apple to ever agree too. Perhaps, they could be pursuaded to add two though:

The back button to the left of the home key. One click to go back. Hold it to go to the map app in 'current location' mode for reduced cognitive load when driving.  Perhaps your choice of navigation apps could be substituted. I personally like MotionX GPS when roaming because it allows you to cache maps.

The quick app button to the right of the home key. The default would be one click to bring you to the camera app, and if you are already in that app, to take a picture. No more need to take gloves off or fumble when a photo op arises. Hold the quick app button to instead bring up the phone app. If you are already in the phone app, it would phone the first item in your contacts. You can now initiate a call home with your phone in your pocket, using your bluetooth device. Or if you wanted you could program this as 9-1-1, so you could call for help unobtrusively. It also ought to be possible to override the defaults to allow you to make other apps your 'quick access' ones.

On iPhones the user always can place whichever apps or web pages they want on their home screen. This is great for most purposes, but not for the two situations I outlined.

I don't think a 'search' key is really justified as a hardware feature. Many apps have no need for search functionality. And global search can be accessed using the home key and a swipe. Although it's unfortunate that not all apps display listings on the global search. Usually if you are going to search, you have to type, so this is not a hands-free type of operation that would justify a hardware button.

I can imagine that when doing an outdoor activity,  I might want to rapidly alternate between map, camera and answering or initiating a call. When driving I might need to alternate between navigation and phone without taking my eyes off the road (assuming that the navigation app speaks out directions). The number of times I have been taken from app to app and then to a web page, without a very quick way to go back, also makes me crave a back button.

Stephen Tenerowicz has his own thoughts on hardware buttons. Design guidelines for Android can be found here. iOS human interface guidelines are here. Windows Phone 7 human interface guidelines are here.