LOL, I'd apologise about derailing the thread, but I reckon that's already happened. :)
If you google "tethering detection", you'll find pages and pages on how networks detect it. So the technology exists and they can most definitely do it. I believe it relies on analysis of traffic patterns.
Funny you should mention giffgaff, the 4th google hit was this:
http://community.giffgaff.com/t5/New...2/td-p/9187628
if my skim-reading works, as of last Summer, auto detection of tethering was activated on giffgaff.
Just because there's been no action, doesn't necessarily mean nobody knows it's happening. Being aware of something, and choosing not to react, is something parents do every day! ;)