Updated March 9, 2013
Okay. I was exaggerating. He actually showed all of us the bottom of his foot again.
Or more precisely, he showed the photographer or cameraman the bottom of his foot, and they shot/filmed it, and then we saw it in a magazine or in a video.
The point is, he frequently lifts his leg to the extent that if he weren't wearing those cool sneakers, we'd actually see the sole of his foot. Of course, all fans know that his signature dance move is to hop around on one foot while showing us the bottom of the other. And maybe most of us noticed that in photographs, he often stands contrapposto, with his weight on one leg, and the non-weight bearing leg held slightly in front of his body. (Michelangelo's David does this, too. Except his non-weight bearing foot is gracefully pointed toe down. And since David is a marble sculpture, and Aiba is an idol-so why I'd be comparing their looks is a mystery even to me-I think David's beauty has nothing on Aiba's.)
Anyway, before I started this post of course I Googled "people who show you the bottom of their foot," because maybe there's a reason why this is happening, right? Except, all I know now (at least from Google's first 20 or so hits, which is how many I clicked on before I got bored) is that if Arashi becomes very popular in Thailand or Islamic countries (aren't they already popular in Thailand?) Aiba might want to be a little careful about this habit, perceived rudeness and all.
Then I developed my own theory of the "foot sole showing" action exhibited by everybody's Aiba-chan. Aiba seems to be honest, open, and hesitates little before jumping in to try things, whether they're scary, or unusual, or likely to end up with him being very embarrassed. When animals are happy and trust us and are willing to be vulnerable, they show us their bellies. Aiba shows us the bottom of his foot. Or, at least, that's what I'd like to believe. You are my sole, sole!
:-)
Update: March 9, 2013 - A recent, unexpected example.
Matsumoto articulates his point; Ohno thinks about something else, Ninomiya contemplates his feet, which may be hurting in those shoes without socks, and Sakurai sits rather professionally on a stool set to an uncomfortable height-a height where the edge of the stool cuts across your butt in an annoying way, unless your legs are long enough, like Matsumoto's and Aiba's, or you hoist yourself up onto the seat and use the stool's footrests, like Ohno. Oh yeah, and Aiba showed us the bottom of his feet, again.