There are a few articles out there on this. One said that o2 will be checking the usage of their cilents and if they suspect a sudden increase it then will contact them to warn them. They also mention the possibility of extra charges/disconnection. From the above it would seem they are unable to differentiate normal iPhone traffic from tethered data so I'll just ride it out and hope I slip under the radar - I'm not going to be using much traffic anyway.