"Because it is no different surfing the net with the iphone displaying the data or your laptop displaying the data."

I'm not sure I fully agree with that statement. Your behaviour is likely to differ between iPhone usage and what you do on your laptop. e.g. on your laptop you're more likely to exchange large documents via e-mail, download Windows updates, etc.

There will be some concept of what an "average" user does and their current tariffs will be priced accordingly. Changing the device being used is likely to change the assumptions.

Controversial suggestion: maybe what you want is a capped usage tariff that can be shared between iPhone and tethered devices, rather the current "all you can eat"....

O2 in the UK will be supporting tethering on the iPhone as an add on to their contracts.

Contracts start at 29.38 GBP (approx 48 USD) for an 18 month contract. Tethering starts at 14.68 GBP (approx 24 USD) extra for a 3GB package.

Details at: http://shop.o2.co.uk/update/internet.html

Except for the bouncing Google logo you get on the IPv6 version.

In this case "BT" actually means "BT wholesale", so the issue applies to any ISP which uses BT's DSL platform. This includes both AAISP (the ISP in the linked article) and Entanet (resold by various other ISPs), the only two UK ISPs I know of who offer native IPv6 over DSL.