Scarcity
sethgodin.typepad.com
sethgodin.typepad.com
Give invitations for free while you are obscure
Grant 50 invitations to the early adopters
Start a bidding process for new adopters that closes every hour.
Thus allowing the early adopters to cash in on their invitations or give them away for free.
You control demand by selling your own unlimited supply of invitations (to cap the price or cap your expansion).
The definition of economics is the study of resource allocation under scarcity. Giving away stuff for free is marketing, not economics.
Incidentally any action taken in a free market can be described in terms of Economics, even when there's no currency changing hands (e.g. opportunity costs on the part of the seller/buyer).
If you're looking for the scarcities in the "Economics of free," I think 2 are pretty straightforward: Buyers' and sellers' Time (current), and the sellers' Product (future) that's certain to follow any "free" strategy.
I'm not one for bans, but I'd request people to take it easy on those Godin posts.