Look here's what the current state seems to be:
You can buy the device separately and dock separately. Who knows how much that will cost. A lot, since the dock alone will be $190 multimedia dock, $500 laptop dock.
You can use the Webtop environment on Wifi without having a tethering plan.
IF you want a discounted price (mail-in-rebate, sigh) on the phone+dock bundle, you MUST purchase tethering.
IF you want to use the Webtop on the AT&T mobile network, you MUST purchase tethering. Some people have said that as soon as you launch Firefox you'll automatically get put on their tethering plan, others that you'll be prompted, I think that's all speculation at this point.
So yes, you can use the device without a tethering plan, if you plan to always be using it on Wifi when docked. Personally that kinda seems to defeat the point of it.