Clicky clicky
Just tried it. Works perfectly. Remember to follow the instructions at the foot of the site to re-enable MMS. The tethering option still stays in place - instead of "Set up Internet Tethering" you have the option of just turning it on or off.
O2 have already confirmed via Twitter they don't have the systems in place to detect the difference between data from a phone or from a computer. The only way they know is by the amount of data being transferred. If it's excessive (like watching BBC iPlayer etc) they will contact you and tell you to cool it.
Just don't use it excessively and you won't have a problem.