1. how do you know it's 10 days?
2. he hasn't done any tethering. That's just normal iPhone data usage.
3. Tethering is not yet supported by AT&T but the appearance of the Tethering category under the Usage section makes me think they will roll it out soon.
I too have the tethering data on my 3G - difference is, mine is a free one from Italy from which the tethering option dissappeared when I started tethering using a USB cable.
For I don't use/need tethering, is there a way to remove the tethering meter? I assume my provider will never allow tethering with my data plan, so I don't really need that...