> There is precious little data to suggest tethering users actually use more data.
There is data, though no carrier would publish those.
You wouldn't fire up HD videos on the small screen of the handset, even if the bandwidth would be available. With a large screen and a real keyboard you have more opportunities using up the available bandwidth. Also there is little motivation downloading ISOs (legitimate or otherwise) on your handset. Even with OTA phones usually don't get 100 MB of OS-patches a week (there are even features to throttle the bandwidth to the Windows and Apple update servers during peak-hours, I kid you not).
Also "flat"-rates which are only intended to be used with handsets are made cheaper so that selling a new smartphone along with a new contract is made easier. As I once saw in a presentation from a large Performance Enhancement Proxy vendor: "Bandwidth is a gas". Give the users the possibility to use it, and they will certainly use it until all capacity is maxed out. Especially in times where carriers are investing buckets full of money to set up 4G networks they are afraid of one thing: That subscribers will use the extra capacity to go to online movie rental shops who're going to make the profit - and extra capacity doesn't necessarily lead to new subscribers... That latter will most probably happen, I saw it happen a few times after large mobile network expansions. (Adding more GGSN to the network, therefore distributing the subscriber's traffic over more internet handovers points than before)
Future tariffs will be
* different bandwidths (256k, 2M, 5M, something along those lines)
* different caps
The whole anti-tethering thing is not greed, but Angst. Sure, mobile phone carriers have a problem with their business models. But a mobile phone carrier ain't just another ISP: They have to run a mobile network only to be ALSO an ISP. And that's expensive. Really, really expensive.