Geoff Baker has a blog entry today on the celebratory move, where he mentions that various Japanese readers have told him that "there has been much chatter about it on TV overseas and that the move is known as "The Komanechi" ... supposedly, it was invented by Japanese comedian Kitano "Beat" Takeshi and is meant to be a silly sort of gesture, one that is supposed to strike up memories of the tight leotard worn by Romanian gymnast Nadia Comaneci."
there is an interesting reader comment from marinersfan1976 in Tokyo which would seem to corroborate the theory:
On Sept. 19, 2009, I saw the Japanese TV show "Joho 7Days Newscaster", which Beat Takeshi co-hosts. The video showed Sweeney yelling "Comaneci!" to Ichiro after Ichiro's 9-year, 200 hits pre-game ceremony. At that moment, Sweeney did the Comaneci properly with both hands, and Ichiro responded with a one-handed hip-chop and leg kick while walking back to the dugout. Ichiro's Comaneci, both then and after the walk-off, didn't look quite right, probably because he was moving while he did it. This may be the source of some of the debate on the blog. However, if Mike Sweeney is yelling "Comaneci", it's clearly inspired by Beat Takeshi's gag.
My two cents from Tokyo