I used to tether before 'tethering' existed. When I was a rep with AT&T back about 10 years ago, we used the data transfer cables they'd send for transferring numbers out of an old phone to a new one to hook to our laptops - then use the internal WAP connection on the phone as a wonderful 56k modem!
As far as today goes though, there's actually a procedure that most carriers use for tethering prevention...
5GB is the target number they use - unless you crank up over that they won't flag your account. Once you are flagged, you will first be throttled (data speeds will be cut) and they will monitor you for 90 days. If you continue to go over the 5GB mark, they will supposedly begin checking the IP's of the sites you are visiting. If they find things like MegaUpload, LimeWire, RapidShare or other Torrent sites, you will get dropped like a rock...
Other than that, with the streaming apps now, there's not much they will do to you - except for charge you for any overages over the "mythical 5GB cap" that they use on their Mobile Broadband Devices. Unless 4G gets here before then, in which case, they may not even care at all with the cheaper prices that affords them.
As far as the 200GB's? 200 GB's is a gigantic amount of data and will get your booted off Comcast, Xfinity, Fios, DSL or just about any other non-business carrier today. I have no clue how you snuck that much data off on a cell connection, but on Sprint I can't even download fast enough to get to that number. I think if I ran mine 24/7 for 30 days, I'd probably be able to hit about 40GB per month simply due to connection speed limitations - could just be the Moment is that slow on the data transfer though?