[Samsung Galaxy Nexus] KWKSLVR:
I don't know how Verizon works, but some carriers, I believe mainly T-Mobile in my experience can technically tell if you are using tethering or not. To put it very basically, when your phone sends a data packet via tethering, that data packet hits a different set of servers on your carrier than a "regular" data packet.

At the same time, it would be very expensive to implement, maintain and regulate a system that monitors every single data packet. In essence, your carrier can tell what you're doing but only if they go looking for it. There's no automated system that flips a switch and says, "That dude just sent a Tethered Data Packet and his account doesn't have Tethering enabled". It would be a huge undertaking to wrap the logistics of it all from the technical department into essentially the billing department and have them talk back and forth to each other. It's one thing if you have a smart phone with no data plan and you hit a website, like on AT&T if you slap your sim card from a clamshell into an iPhone. Reading that HTTP traffic is easy. A system set up to say, "Whoops, that dude doesn't have a data plan, you just accessed data, time to add one on" is easy. It's another thing to inspect things on the packet level. It's "Data/No Data" vs "Data Type 1 vs Data Type 2".

I tethered on a regular basis on my old EM+ plan on T-Mobile with various SGS Rom's that had WiFi tethering from the international Galaxy S enabled, and never heard a thing from them. I've been doing the same on a regular basis with my GNex too. However, I'm doing mostly web browsing and remote desktop connect. I also don't really abuse things, not even within the limits of my data plan. I have 10 gigs a month, and right now I'm almost halfway through my billing cycle and I've used 2.3 gigs. There's not a lot of stuff that I hit that you couldn't really do from a phone. I think it's relatively easy for them to monitor http traffic, but inspecting data packets takes a lot more work and I have a feeling that Verizon doesn't have much interest in spending the money to "crack down" on the relatively few people that even own a GNex in the first place.

In general, I have a problem with rouge tethering when it's being flat out abused and ends up raising costs for everyone else. You know, like people using 100 gigs a month and stuff like that. Seeing how I'm paying for more data than I use, I don't really have a big guilt trip about it. $30 to enable a feature like WiFi tethering is a little crazy in my opinion, especially when most carriers seem to be charging separate data rates for essentially the same chunk of data. If "Mobile Hotspot" was $10/month, I'd turn it on.

My understanding, and in my observation of a lot of people that have been busted for tethering, they were "abusers" that ended up flagging their carrier. And/Or, their carrier (like AT&T) started monitoring App use and flagged activity based on the app more than anything else - like this, which, by the way, is that app that I'm using right now on my GNex, on Verizon typing this on my laptop. ;) Blocking the app from showing up in the market is a MUCH easier/cost efficient method for carriers to implement than trying to sniff every data packet. When you think about it, the vast majority of smart phone users on Verizon, are not GNex users. The vast majority of GNex users probably aren't even rooted, much less know how to install an app from any other method other than the market. We are the leftovers, and Verizon doesn't care as long as you aren't on an old unlimited plan, use 50 gigs a month, and 48 gigs of that ends up being "Hot Spot" activity that you're using by rouge tethering. If that's the case, yeah, I'm on Verizon's side from a moral standpoint. PAY for the feature (from a moral standpoint, not company policy).

On the other side, if you're using 50 gigs a month and it's all NetFlix and Podcast downloads, and 2 or 3 gigs is rouge tethering, then Verizon can get over it. It's an "unlimited" plan. Scale their speeds, don't raise the bill.