The only said that the iPad version would be out in the coming weeks. While its likely they won't include that as a feature, we can't really speculate yet considering the iPad version wasn't even demoed or discussed.
Highly unlikely will the iPad receive such a goodie.
1. It would have been demoed for at least five minutes in the Keynote as it'd greatly enhance the functionality of the iPad.
2. most WinRT vs. iPad ads list this "snap" mode a plus. (And they are right, I should add. I've used WinRT tablets and found the snap (dual-pane) mode absolutely great.) Apple could have easily stopped those ads by simply announcing / demoing the iPad's similar mode.
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Yes, it could be something they're keeping secret for this Fall. We shall wait and see...
as for the above, split screen should not be an issue at all. A friend's android does it.
Highly unlikely. As I've pointed out above, Apple could just make WinRT much-much less desirable by demoing something similar. Now, a lot of folks wait for the next-gen and, hopefully, higher-res screen Surfaces because of the snap mode only. Apple could have easily made the iPad for these folks as (or even more) desirable as the WinRT tablets. Apple may know very well they will be losing a lot of customers to WinRT because of the snap mode of the latter.
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That's a Windows 8 feature. Apple would have been accused of stealing it if they implemented it into iOS 7. And something like that makes a lot more sense for Windows tablets because the screen sizes are so fragmented that apps have to be designed to work in different spaces. On the iPad, every single app is designed to work in the same dimensions so they aren't as flexible to running in obscure sizes.
You're right in that, when programming RT apps, you must use a given pixel width (320 pixels) for the snap app (the one in the left pane). That is, you cannot display arbitrary content there - there is no way of, say, dynamically downsizing an otherwise full-screen 16:9 view there.
However! This doesn't mean a full windowing system, like that of the absolutely excellent windowing tweak Quasar, couldn't be implemented by Apple. Then, the system would always just downsize the originally full-sized 4:3 windows. Two example screenshots I made running several apps at a time in Quasar on an iPad 3 (warning: large Retina-screen screenshots!)
http://dl.dropboxusercontent.com/u/13100693/html/042012RetinaHDVideoPlayers/jb/0526Quasarshot.PNG
https://dl.dropboxusercontent.com/u/81986513/092012/ASFWMVPlayer/IMG_1720.jpg