The way I've always understood/interpreted it: unless you are using what AT&T calls a "smart phone", you get MediaNET. If you have a smart phone, you get a data plan. It's through the same 3G network, but I think there's some proxy-ing going on.

The data plan allows for tethering, if you pay for the privilege.

Tethering with MediaNET "works", mainly because AT&T doesn't seem too interested in locking that function out in the consumer phones they sell. However, under the TOS, you're not supposed to use MediaNET for anything other than surfing the content managed sites they provide, usually to their site for buying ring tones, wall paper, etc.

MediaNET Unlimited is really MediaNET 5gb, but that's a lot of data.

Final analysis: Let your conscience be your guide. If you feel it's worth it, get a 3G consumer phone (I have the LG Shine, and it's great at tethering) and tether it to your N8x0. Just understand that it's not officially allowed.