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.