Incremental advancements will make them -better-, but current generation devices are already there.
Every single person I've seen get a Blackberry, iPhone or android device has seriously curtailed their laptop usage. They've also (almost annoyingly) increased the degree to which they reach for their mobile apps, for tasks that could never justify pulling out the laptop.
It's reminiscent of the way that early laptops, while not yet sufficient to replace any desktop, still significantly ate into desktop usage.
The 'tipping point' he postulates is yet to come has already passed. In my opinion, Nokia's n770 was probably the last device that wasn't -quite- there. Everything after has been plenty capable hardware-wise.