T-Mobile state that you are not allowed to tether in their terms and conditions, but I would imagine that is more to discourage people using their phones as their only internerd connection more than anything. I believe they also attempted to get Google to remove a load of tether apps from the Android Market too… not sure if that's still the case, so they don't like it too much. But as stated above, I don't think they can tell you are doing it, unless you start tunning torrents and like through it.
O2 charge as has been stated above…
Vodafone are quite happy for you to tether, but their data-allowances can be quite low..
I've got no idea about Orange…
Oh, and nothing wrong with HTC, I got a Desire on Saturday and it's very lovely indeed…