I think you're missing the point. People complained that "unlimited" didn't really mean that the service was unlimited. Now they have a 2GB plan instead, but they clearly intend for you to not use that much data.

Yes, a desktop OS will likely consume more bandwidth. But there's still the 2GB cap. If you're still under 2GB after attaching your computer, why should they charge you more?

And the only possible answer is that they don't want you to actually consume the 2GB of bandwidth that you've paid for. Basically, you pay $25 for 2GB of bandwidth, and then you pay another $20 to actually use it.

I understand where they're coming from, though. They want to put a cap on the amount of data you're using. They don't want you to use your 3G connection as your home ISP. But they also want to avoid penalizing people who occasionally go over whatever their real maximum is.

The correct solution to this is probably something like: Set the cap at whatever their real limit is. Let's say 250MB. Then have a floating window of free overage quota, say 6GB over a year, 500MB a month. Each month you go over 250MB, that overage gets subtracted from your quota. After a year, you recover that used quota.

It sounds a little complicated, but it's actually easy to depict on a bill. Have a stacking bar graph that shows each of the last 12 months of overage, and mark the cap. (See below.)

The somewhat odd part is that when you go over, you have to buy more quota for that span of 12 months, so that you don't get double-charged. That is a little harder to depict.

I understand that people are suspicious of complex billing schemes, but something like this probably is the most fair for everyone.