Now - these people quite often want to know when you launch, and the best way is to give them the website address, so they can leave their e-mail address on it.
So: promoting a landing page? Makes no sense. But talking about the idea before you have a prototype - definitely!
2) there are people who you have shared the name with; if they do type it into a browser, they should be reaffirmed that they remembered it correctly etc
<shamelessPlug>My startup, http://www.loggur.com, says we're launching soon but I actually did take a few weeks to describe it in as much detail as I can on the site. There are at least a dozen pages of details.</shamelessPlug>
However, you did hit the nail on the head; it's better to have a viable product to show or something to offer to potential customers/users in exchange of their email addresses.