It seems it is possible add Database.com users into a 'normal' Salesforce.com org (i.e. a standard EE or UE org) to allow large numbers of users cheaper access to API and OAuth based authentication.
The documentation at http://ap1.salesforce.com/help/doc/en/users_understanding_license_types.htm remarks "Access to data is determined by organization-wide sharing defaults."
So, my question is:
If the OWD is set to Private and a Database.com light user creates and manage their own records - can those records also be shared to internal Full License users for use in reporting, standard page layouts and Visualforce?