T-Mobile UK now charging for tethering workaround iPhone

I have been using tethering for a few months on my iPhone 4, mostly for email access and limited internet browsing.
Yesterday I got a shock. I hooked up as normal but instead of being able to browse, I got a T-Mobile intercept page forcing me to but Pay-As-You-Go mobile broadband access. £15 a month or so, which would double what I pay today.
It seems T-Mobile wants to jack up charges to its users having just lost its case to slash monthly limits from 3Gb to 500Mb; hence the introduction of this new set of charges.
However, Im intrigued as to how they do this. I had previously thought that all T-Mobile can see is a datastream and cant tell whether you are using your iPhone or your Mac (although you can guess based on the amount of data consumed). Im guessing that they are looking at something called the user agent string, which tells you things like OS/Browser etc and I suppose could be used to define when you are using the iPhone and when youre using the Mac. Im guessing, too, that it could be pretty easy to spoof the user agent stringso T-Mobile think you are using the iPhone when in fact youre tethering.
I guess that theres a possibility that all the networks will try this on as a way of increasing their revenues. Pretty rich when Ive bought - in theory - free, unfettered internet access as part of the package from them.
Whats the prevailing wisdom here about whether there are sensible and effective ways round T-Mobiles latest attempt to gouge more out of its customers.
Richard
iPhone 4 iOS 4 WinXP