Agreed it's an issue - considered including it but didn't.

It's fair to question how much developers should concern themselves with external costs for using their service.

Do we have a responsibility to design for data caps to prevent unnecessary charges? Do we have a responsibility to do the opposite, as an industry exerting pressure on carriers to increase cap levels? Somewhere in between?

Honestly, I don't know. I'm inclined to design for a user's reality rather than a principle, but that "in the real world" thinking allowed the web to be dominated by "best viewed in IE6" sites.