Tim Bray suggests metered billing for mobile data is the way to go. Me, I think it is a terrible idea for psychological reasons, and it would be a serious threat to companies like Google that thrive on people freely using online resources. It'd push people to block ads, use local apps over on-line services and generally limit how much they do online.
For psychological reasons it's better to go with pre-paid data rather than metered, and to make sure that going over the limit does not incur extra, unexpected charges. Here is my Google+ post with much more detail: Metered Data and Prepaid Data
This leads to a separate question: What am I to do with posts like the one I have on Google+? On one hand, it's pretty long so it fits better here on the blog. On the other, the people that have circled me on Google+ are much more likely to be interested in this sort of thing than you here. It feels kind of wasteful to double-post the same text on both places.
For now I'll try doing thing like this: post longer bits either here or on Google+, then give a short summary and a link on the other place. If anyone have better suggestions I'm all ears.