Re: multi-finger pinch to replace "home button"
That gesture just seems a bit awkward and non-intuitive. It could eliminate moving your hand off the screen to hunt for the home button, for users who actually learn and remember the gesture. A time-saving short cut for power users. But probably not a replacement for the home button. Just imagine the frustration for users who haven't been trained to 5-finger pinch yet and can't turn on an iPad.
Re: physical volume up/down buttons
I'd expect the buttons to stay. Just look at the non-iOS iPod line for hints. Apple seems to use the iPod line (especially the nano) as their fashion-forward user interface testbeds. For example, the previous-generation shuffle lacked any buttons. Users were expected to use the control built into the earphone cable. And apparently it didn't sell as well as before.
So Apple went back to the circular button arrangement for the current generation shuffle. It's almost a skeuomorph for the original iPod's click wheel. It's familiar, iconic, and easy to find and use without looking.
But the cable control requires a little more fumbling and fiddling than on-iPod buttons require. You need to find the control on the cable, then twist it so that one of your finger pads is touching the clickable surface of one of the buttons, then squeeze. You can't operate the buttons just by squeezing from any direction. I think it's less convenient and slower.
And if you are listening without earphones through the built-in speaker, what are you going to do? Remember to swipe up and down on the screen or something? And what if your iPhone is upside down in your pocket? Would you need to find the end with the earphone cord connector and remember that that end is the "top"? And what happens when Apple finally gets rid of the earbud cord? It could happen soon.
Re: physical buttons required for easy restarting
That's the real killer, isn't it?
So I'm just not convinced (yet) that there is any compelling reason to eliminate the physical home button. It's not an expensive component. It fits in the iPad's bezel, which can't be eliminated. The bezel is necessary to prevent thumbs from inadvertently tapping or swiping the iPad's screen, so the home button will always have a place. The home button could even be made somewhat smaller to save space, without making it harder to use.
And how different does Apple want to make the iPhone and iPad? iPad with bezel, iPhone without bezel? That might confuse users who switch between the two devices.