My initial reaction to this is: yes, please. Four-finger swipe on the iPad is already my new default way to bring up the Multitasking Tray — it just seems much more convenient than double-tapping the Home button.
But then I think about 3 issues: (a) How infrequently I use the Multitasking Tray on iOS; and (b) how often I launch the apps on my iOS Dock by sliding my thumb onto the bottom of the screen; (c) how easy it would be to accidentally bring up the Multitasking Tray.
The third issue is the one in particular would be the problem for the vast majority of users. Ironically, the whole point of this linked-to concept video is to demonstrate an easy way to launch the Multitasking Tray, but I think the ease is exactly the problem.
The bottom-third of an iPhone is surely the area that gets the most touch input. It’s where the Home button is, the Slide to Unlock button, and the Dock of icons. How many iOS users regularly access the Multitasking Tray so often that a slide of their thumb onto the screen would be that much more convenient. I imagine it would bring up the Multitasking Tray far more often that desired — even by power users.
And Ken Yarmosh agrees. He had the gesture activated on a jailbroken device and found that it caused interference with using several apps. Surely a system-wide implementation by Apple would be better than a jailbroken implementation, but it’s very possible that the reason this gesture doesn’t exist on iOS is because Apple hasn’t found an elegant way to implement it.