Agreed, the underscore in the data-attribute isn't ideal. I initially tried to use camelCase, but alas, we often forget that DOM elements are case-insensitive:
Or you could say I'm not forced to use it, but now I'm using camelCase for variables and underscore_case for data attributes... so the two don't match, so why have the non-standard underscore_case at all.
(This would be an example of one of the things that makes libraries feel cludgy, and one of the reasons I love how consistent Backbone is, like I was talking about further up the thread with Jashkenas.)