In the official iPad Mini video, Ive explicitly says "...you can still pick it up and easily use it with one hand..."
Easily!. Earlier in the video, they make the distinction that you can
hold it with one hand, but at the 2:10 mark, they're clearly talking about
using it with one hand.
Video is here:
http://www.apple.com/ipad-mini/overview/#video-ipad-mini-features)
Going by that reasoning, any device can be used one handed, yet according to their iPhone 5 campaign, one of the main talking points is that anything larger than a 4" smartphone is unusable with one hand.
So either the iPad Mini can be used one handed (like it says in the video) and therefore so can other smaller smartphones, or nothing above the iPhone 5 screen size (like it says in the iPhone campaigns) can be used one handed in which case the iPad Mini video is a lie.