Just to add on the tethering WiFi, in the past your phones serial number would be the only device that could be used on the Cloud/BT network. From the update, you are going to be able to use two devices on the cloud network only, meaning BT won't work while tethering the device.
Preduk,
Not sure what your saying about the above about 2 devices and BT Wont work. Please clarify...
My understanding from what I have read (and I could be wrong and its late now) is that you pair your device(s) to the iphone via either blue tooth or wifi (or both) and that you use your iphone as the modem/access point - which then decides whether to use the 3G connection or the WiFi connection if available to access the net.
According to the O2 link I posted above on tethering it is Cloud/BT Openzone and partners that the unlimited access is on with you paying based upon 3g/cellular allowance.