For all you wondering how they can tell:
All IP packets have something called a TTL associated with them. It stands for Time To Live. Every "hop" along the network from one router to the next reduces the TTL by one. When it reaches 0, the packet is dropped. This was introduced to keep routing problems from overloading the network. If for example, by some error a packet was going around in a circular path, the TTL would eventually reach 0 and prevent a packet storm.
The thing is, ALL routing devices do this. OSes use standard TTLs. For example, let's say both your iPhone and laptop use 127 for the TTL. AT&T will receive packets from your iPhone with a TTL of 127, but since the packets from your laptop pass through your iPhone first, they arrive at AT&T with a TTL of 126. They can detect a tethered device this way.
Apple uses a TTL of 64 for the iPhone, by the way. So change the TTL on your computer to "65" and there should be no problem. Here's how to do it:
1. Click Start - Search and type “regedit”. This launches the WIndows Registry.
2. In the registry, navigate to the following registry key [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters] HKEY_LOCAL_MACHINE
3. In the right pane, right-click and select New – DWORD (32-bit value) and set its name as “DefaultTTL” and set its value anything between “0? and “255?. The value sets the number of Hops or links the packet traverses before being discarded.
Kudos to Ryan Laster1. I don't have an iPhone to test this.