Currently running three sites off of the same database:

www.PimpCostumes.com
www.PlatinumCostumes.com
www.CostumeShoes.com

Not a trivial customization, but doable.

Basically each site uses its own install of ZC directed to the same database. Most tables are shared. Configuration and site specific tables are unique, by adding a _sitename to the table name in tablenames.php with the required tables created. With this method you can share orders and customers or not.

As each of my sites has a different category and product layout, I created a top level category for each site, then modified the the category and product SQL to only look under the current site's top level category.

Products can be easily add to one site in the admin and then copied or referenced to the other sites.

Sites have been up in this way for a couple of years and several ZC upgrades.