I think I'm in the same boat, tethering just doesn't work and it probably never will on the old tariffs. I contacted O2 about the same thing last week but they never responded, that's customer service for you. I'm guessing they will say the same as with you anyway.
I wouldn't hold onto your "unlimited" tariff just because it's "unlimited" for the reasons below. O2 essentially do not honour the contract they originally signed you up onto and when there are problems like mine below they don't seem to be able to sort them out.
My "unlimited" data bolt on didn't stop O2 from blocking me for almost an entire month late last year - I essentially could not use any data for almost a whole billing month. I was syncing my phone with iCloud and unfortunately my broadband router went down overnight meaning the sync was mostly over the O2 network. Nothing I could do about it unfortunately. I got a text message off O2 the next morning saying this:
"O2: You're using a lot of data this month, which my start to worsen the experience for our other customers. If you continue to use a high amount of data this month, we may have to slow down or stop your data allowance until your next bill day. If we do this we'll text you. To avoid this, we recommend you use Wi-Fi."
Erm... but I'm on unlimited. I understand the fair usage policies that were introduced after I signed a contract with O2 but I checked and I had used just over 4GB. Hardly even close to what I think many would consider a fair use of "unlimited". Then just with normal usage of emails and iMessage over the next day or two I got this:
"O2: You've used an extremely high amount of data this month, which has started to affect the experience of our other customers trying to use data. To be fair to all our customers, we've been forced to slow down your data speed until 21/11/12 which is your next bill date. To experience a faster data speed, we recommending using Wi-Fi until then."
I thought ok they may reduce my speed. I can live with that for a while. But despite this stating I was just on a slower speed, nothing worked at all. Every app or anything data related just timed out. I contacting O2 several times and explained the problem to them. At first they told me there was a block on my account and they had removed it and to wait a day. Still didn't work after a day. Then the next time they told there was no block on my account and to check my phone and network settings. I explained that it worked perfectly right up until I received the message from O2 that they were going to slow me down. The O2 support employee I chatted to on the online support tool basically could not help me and was arrogant and argumentative, saying things like "You need to believe in what I'm saying". I have the transcript from the O2 chat tool to prove this (it emails it to you).
They clearer didn't understand the technical processes they were asking me to perform and wasn't really listening to my problem. All they were suggesting was changing my network settings when that was obviously not the problem. I obliged and went through various network setting changes and resets but nothing helped as I was clearer blocked from the network side. I was trying to show that I had done all the necessarily steps to rule out the phone and settings. I asked for second level support number and all they could give me was the standard 202 number when I had already spoken to first level support two or three times.
I gave up in the end having got exactly zero help from O2 support. And yes, there was nothing wrong with my phone or my network settings because on the exact day of the next bill date from the text message above it all started to work fine again. The text message from O2 stated I would get a reduced speed, not zero service. And that was with what I would not consider close to a huge amount of data on an unlimited tariff (just over 4GB).
