Are you sure she's dancing? Maybe she just had a look at her stock portfolio. Anyway, how is this any sillier than that guy who dresses up like a stormtrooper?

Yes, she is dancing and has been at it for several years. She used to be a common site in Akihabara before they shut down the hokoten after the stabbing spree.

I like this other clip someone shot outside Shibuya Station where a bystander became her prop. Watch where her hands go.

http://www.youtube.com/watch?v=FeB_HRn5Crs