I haven't read much about how carriers feel about this.
For example: Will it be the end of tethering fees? Because root privileges on an HTC EVO allow you to tether wirelessly, circumventing an addition $30/mo, will Sprint just stop charging for tethering? Or will there be another level of restrictions in place to prevent it?
If they can find a way to charge you, they will.
I guess they'll start looking into various other checks such as browser/user agent when visiting websites.
The speculation about how they do it is to measure the TCP TTL. If it's off by one from what you expect, then the packet originated with a tethered device, rather than the native device.
I'd like to see them try that with me. All of my phones data traffic bounces through my Linode box thanks to a rooted HTC Desire Z running OpenVPN on CyanogenMod 7.
It seems like this might be moot soon since unlimited data seems to be going the way of the dodo anyway. Assuming the T-Mobile merger goes through, Sprint will be the last carrier to offer it.
AT&T had also started cracking down on people tethering without paying, with the punishment being losing your grandfathered unlimited data. I imagine this wouldn't be hard for Verizon to do either, now that they've got data caps too.
