In my mind, this basically confirms an iPad Pro is imminent. Split-screen will be nice on a large 9.7 inch screen, but the clear purpose of this feature is to make an even-larger screen worthwhile. Crucially, I expect that developers will be able to customise the app UI’s for the split view appearance, maximising usability.
Of everything that has been reported about iOS 8, this is my most anticipated improvement by far. I’ve been theorising about this kind of thing for months and have been hoping that this is in the works.