I've heard that they do deep packet inspection so if they see e.g. a desktop Chrome user agent, they know you're not on a phone. Sounds excessive, but it could be true. (And yes, you could probably find ways around that, but I'm not sure what the goal is here.)

If you want a portable data service for your laptop, this is not the plan for you. I was using Clear (Sprint WiMAX) for a while and it was relatively cheap and fast. Only downside is the limited coverage area -- check the map before you buy.

I have heard rumors that they use the TCP TTL field to detect tethering, regardless of device type. If you open a connection from a tethered device, the TTL field will be one count smaller than if the connection originated on the device itself. (This assumes that all devices being used have the same default TTL).

Ref: http://netmgt.blogspot.com/2011/03/detecting-obscuring-tethe...