To enable tethering you have to activate this feature with O2. If you used the O2_UK.ipcc file (which was doing the rounds during the Beta) with your newly updated iPhone 3G running 3.0 GM seed this would enable tethering without notifying O2.
As long as you didn't go crazy with your data then I would suspect that they will never know.
You would need to update the carrier file on your phone using the o2_uk.ipcc carrier file.
You can do this through iTunes by pressing option or cmd (or ctrl on Windows) when clicking the check for updates button I believe (I can't remember 100% now).
People are still using PDANET - and I have not heard any stories of the O2 police coming to arrest them.
There is a possibility that the 3.0 software handles tethering differently and flags every tethered data packet with some kind of "Stolen Data" message.
But it's not very likely.
I guess it will require some brave souls to try it out and see what happens.