I think its only availalbe on the Chocolate 3 right now. I could be wrong. check vzw.com and type Dashboard in the search box.

It's only on the Chocolate 3, to the best of my knowledge. It's definitely not on the Dare. And no, it won't cost you anything extra if you're on a Premium plan. Really, it can only cost you MB usage, as I don't believe that any third parties have created any paid channels yet. (Hell, I haven't seen a single new channel since it launched.)

For the record, Dashboard replaces Mobile Web as far as shortcuts go. To access Mobile Web, I believe that you HAVE to go through Dashboard, which is somewhat annoying.