AT&T has announced that they will end their unlimited mobile data plan next week. It doesn’t sound like the result will be bad, though: the outgoing unlimited plan is $30 per month. The new plans are $15 per month for 200 megabytes, and $25 per month for 2 gigabytes — both with reasonably priced options to add more if you exceed the limit.

This all comes with some meaningless estimates about what you might be able to do with that much data:

The lowest-priced data option is called DataPlus and will cost $15 a month. It gives mobile phone subscribers access to 200 megabytes of data each month enough to send and receive 1,000 e-mails without attachments and an additional 150 with attachments. The plan would also offer access to 400 Web pages, the ability to post 50 photos to social Web sites and watch up to 20 minutes of streaming video through the mobile phone.

That’s working with averages, of course, and it all depends upon how big the attachments, photos, and web pages are. The picture I posted in my personal blog this past Sunday is 50 kilobytes, the version you get if you click on it is 270 kilobytes, and the original on my computer is 1.4 megabytes. Google’s home page is about 120 kilobytes, the IBM home page is currently about 870 kilobytes, and this page of photos is almost 3 megabytes, because the guy who did it doesn’t understand the difference between posting thumbnails and using HTML to scale the full-sized pictures. It’s all wildly variable.

What’s more significant are their figures on the numbers of current customers that fit into the options they’re offering. They say that 65% of their customers use less than 200 megabytes per month on average, and 98% use less than 2 gigabytes. Of course, “on average” means that some of those customers exceed the new limits during some months, but even in that event, a 200-megabyte customer who has to pay the $15 overage to get another 200 megabytes is no worse off than before. And when she uses less than the limit, her data bill is cut in half.

This seems like a fair fare, I think. The only part that strikes me as ridiculous is the $20 per month extra charge for tethering. If it came with some extra gigabytes, it would make sense. If you had to pay extra to tether your computer on an unlimited data plan, it would make sense. But, well, 2 gigabytes is 2 gigabytes, whether you use it directly on your iPhone or you use your iPhone as a modem.

The Europeans are used to limits, and paying for what they use. AT&T is starting to change the model here. Will the other companies follow? I still have an unlimited data plan on my BlackBerry, through T-Mobile. I wonder if that will change over time.