These gestures do mean that you don’t have to use the Home button. But there’s a serious discoverability problem with them. The physical Home button is impossible to miss. That it is the one and only button on the faces of these devices is a big part of why normal people are able to pick them up, start playing with them, and figure out how to get around with no help. How in the world would a normal person figure out or guess that they need to do a “five-finger pinch” to get back to the home screen?
Bingo.
Another reason it is unlikely to happen this year is that new products are probably just a few months away from shipping, and likely to begin production in just a couple weeks, and have had their designs locked in for some time now. It’s doubtful Apple would ship the iPad with such a radical change in user interaction without thoroughly testing that the multitouch gestures work as well as (or better than) the various home button clicks.
Apple tests things before popping them into beta firmware. I don’t think the home button is going anywhere anytime soon, but if Apple is planning the next generation of hardware to go button-less, I’d bet my next paycheck that its been in the works a lot longer than any of us have known about this new gesture system.