On the tethering issue, I look at it this way.
Compare tethering to a meal at a restaurant.
If I order their "Big Breakfast" (aka, say $29/2Gb) off the menu, they will bring me lots of food, but, it's a finite amount of food. I should be able to share that food with as many other people(aka, shared devices) sitting at my table as I want. After all, I paid for the food, the restaurant got their money, they shouldn't care how I share it.
However, if I order the breakfast buffet (aka, $?? for unlimited data). Then it's not really right for me to make extra trips, or bigger trips, through the buffet to feed others(aka, other devices) at my table.
Of course, in reality, I'd love to have one unlimited data plan, that I can use my Droid and my Xoom both on. (eg. Unlimited buffet, pay for one, but everyone eats).
Really, since this is really for one person though, I suppose it's more like 2 people at the buffet, but only one plate. So, two people are eating, but only one person has the plate at a time.
Ok, enough food analogy, it's making me hungry....