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AT&T seems to be the most aggressive about looking for tethering, and are quick to detect it. I'm not an AT&T customer, but my carrier charges extra if you want to use your device as a hotspot, and removed the function from the phone. Soooooo, I rooted my phone, sideloaded an app that enables it to work as a WiFi hotspot and uses a proxy server to keep it somewhat on the "down low" and only use it on the rare occasion when I am away from WiFi and really need to get online with my laptop. I probably end up needing to use it once every 2 or 3 months and stay well under my data limit, so my carrier either hasn't turned its all seeing eye upon me, or that little amount doesn't bother them. I'll take the chance, although I'm sure that they would prefer I pay them $30 a month extra instead.
If a carrier really is looking, I think that they will pick it up, often through TLL values or the APNs that tethered data traffic uses. Like Christopher Nerney pointed out, it isn't as if the carriers are new to data analytics. It's just a little unclear exactly what raises the red flag.
The Verizon thing was somehow related to the specific spectrum used or something (I can't remember without looking it up), and it doesn't apply to other carriers. So if you aren't a Verizon customer and you are tether, you are still a bad boy who plays by his own rules. And probably violates your TOS, which I might feel bad about if the carriers didn't constantly change the TOS to be as beneficial as possible to them.