There's a greater likelihood of OSX coming to tablets than the wholesale rewrite of iOS that would be required.Indeed, but it has to be accepted that it's probably not possible for Apple to advance iOS to benefit from improvements like that, which Android and Windows tablets can do (two apps side by side to some degree) without a shift in aspect ratio.
Either Apple are never going to move iOS on in this manner (and as CPU's get faster it's going to be more and more restrictive, when you have the power to do it) or you are going to have to alter the screen to allow for this.
Weight distribution and concepts like leverage and mechanical advantage show that a 16:9 (or 16:10) tablet is more difficult to hold one-handed than a 4:3 tablet.As always, I see 16:10 as giving you MORE screen (but some still can't see it this way and feel it's taking screen away from 4:3)
Computers had overlapping windows before 16:9 was popular. Heck, even 7" netbooks managed just fine. I hope that Apple sticks by their 4:3 AR for iPads if for no other reason to give people a choice. There are no 4:3 tablets currently being produced (at least none that I can find).