Is it importtant that every table in the database have a primary key for which a persistent class mapping is desired? Can we have a database table that does not have a primary key and map a class to it without the <id> attribute?

As far as I know the <id> attribute is mandatory. In other words, NHibernate like any other O/R mapper is an "object-relational" mapper. This means that you need a table with a primary key otherwise that table is not part of a relational database.

Regards,
Radu

AFAIK, <id> is required, you don't have to map it to a property in your entity, but it must be mapped to a column in your database...