Camera shutter buttons have a two-stop action. Half-press them to lock focus and aperture settings, fully press them to take the picture. There’s a delightful tactile indent at the half-way mark so that your fingers know what’s going on. Let’s borrow this two-stop action for the home button. Press half-way to go to the app’s main screen, all the way to go to the phone’s main screen. If you need to fully escape mash the button. If you just want to head back to the main-screen of the app, tap lightly. You can easily convert a light-press into a heavy-press mid-action. It’s as naturally a mapping as you are going to get.
I agree with John Gruber’s response. In my own words, “Novel, but… barf.”
My chief complaint about Android, after a couple weeks with it on the Samsung Galaxy Tab, is the buttons. I feel like stuff that you can only get to by hitting the menu button is effectively hidden. Context menus have their place - and clearly this is all subjective, but in a mobile/touch OS they’re a crutch. A junk-drawer for bits of UI you forgot to include in your design.
Not to mention, the buttons don’t work when they’re not lit up, at least on the Tab, and you have to touch them to light them. So, hitting menu to begin with requires two taps if the buttons have gone to sleep, then you get either 6 options or 5 options and a “more” button. Blech.
Oh, and by the way, I have seen plenty of people fail to use the half-press shutter button on even the simplest of point-and-shoot cameras. Some people just never pick up on that feature.