I made a plugin for MiddleGen awhile back to generate NHibernate hbm.xml files from the database schema. I design the database -> use migglegen with nhibernate plugin for the hbm.xml -> hbm2net for c# class gen. I updated it a bit. Hope it works for you.

Heres the sample task you need to put in your ant file.

http://www.coolniche.com/middlgen-nhibernate-plugin-0.5.zip

<middlegen
......
>

<table name="AppUser"/>
......

<nhibernate
destination="${build.gen-src.dir}"
package="${name}.${tier-namespace}"
assembly="${name}">

<hibernateDAO getHibernateSession="MySessionFactory.OpenSession();"/>

</nhibernate>

</middlegen>