From the new update, when you make your iPhone a dongle you will have one added Mac address for the laptop/computer of your choice on the Cloud network.
PredUK I think what your saying is that your internet access through the Iphone itself will be represented on the WIFI as one MAC address and the internet access via the tethering side is another MAC address. This makes sense as the routined portion of iphone being able to be identified differently as this would allow seperation of devices traffic.
What is not clear is how many devices can be tethered at the same time, I read what you were saying as one and after you have tethered one you wont be able to tether another - however from the links text to me by 02 it appears you can have devices connected by wifi and blue tooth which also implies (and is not entirely clear) that many devices can be tethered at once.
Will have to wait and see I guess.