This seems like it really depends on the way Apple implemented scrolling, where you can pull past the end and then have it spring back. I don't know if this was new with Apple though.
I don't know whether it is new or not, but Apple has patented it.
It is one of the patents Apple has been pummeling Samsung with in Europe, since they implemented spring-back scrolling in a few places. I remember the photo gallery, but I think there might have been another patent involved there. This patent is also the reason that stock Android (and most non-Samsung skins, I believe) flash a light when you hit a scrolling boundary instead of springing back.
Twitter presumably has some sort of license (via the iOS SDK or whatever) to use spring-back scrolling in the iOS Twitter app. However, there is also spring-back scrolling in the Android Twitter app. I wonder if Twitter has a license for that or if Apple is merely letting it go because of their current friendly relations.
