They look at usage. Someone at Apple or ATT probably did some experiments and found out what 'normal' usage was for a month. If you use a lot of data, a lot more than that threshold number, then that makes them suspicious, and in the contract it says something about them having the right to limit or cut off your data usage at their discretion, as for prior notice, I am unsure about that, but they have to right to cut your access to their network.
If I listen to AOL radio, watch Youtube, or stream Pandora all day at work I would bet that I'd fall into the "higher" usage group of iPhone users. Maybe not 5GB, but more than your average users.
That said, given that NetShare only allows SOCKS compliant applications (HTTP browsers, Adium, AIM, etc.) the amount of traffic *most* Netshare users will be relatively small or in line with whatever normal iPhone data usage is. Yes you could drag a 1.5GB iTunes movie over your 3G/EDGE connection but your iPhone would be useless while you did this. I don't think that *most* users would choose to perform large file transfers this way.
My first argument is that if you load a web page that is comprised of 2MB of text, graphics, etc. via Firefox on your laptop or Safari on your iPhone, there is no difference to AT&T's network load. 2MB is 2MB if it is Safari or Firefox.
Second, I make the argument that the NetShare application is not tethering. Tethering is placing my laptop's IP stack on AT&T's network and thus using all ports, protocols, and services available to my Mac/PC. Like using an AT&T aircard. NetShare is an iPhone application (that I installed via AT&T's partner Apple) that just so happens to be controlled by my laptop. Given that NetShare is a proxy, I instruct NetShare to fetch web site to the iPhone that are then stored in the iPhone's memory then passed via wifi to my web browser where they are rendered. AT&T sells a "tethering" service and NetShare is different than that.
So IF one day I get the call from AT&T that I've been "using too much unlimited data" I feel compelled to ask:
1. It is unlimited. There is no such thing as "too much". (Yes, the same ol tired Comcast argument).
2. Prove it. Show me evidence that I was doing anything that goes against the AT&T TOS.*
3. Even if they do flag my account, I will fall back on the fact that I was merely using an application that was promoted to me via their bed-buddy Apple via the App Store.
* Yes I realize that they don't have to prove anything. However the above give you a great arguing position if you handle yourself right.
So, given that my personal usage case doesn't involve going anywhere near 5G (even with a full 8 hour day of heavy web surfing, AIM, eMail via Gmail) I don't think my data usage will fall out of line with most iPhone users. So I'll continue to use NetShare when I am in a pinch and not loose any sleep over it.
Note: as soon as someone ports slirp or some other full-IP-stack "tether" this is all moot. The kids will fire up torrents and ruin this for all of us.