Interesting project, maybe we can switch to it from hbm.xml. The advantages are obvious:

... read more on a web site http://fluentnhibernate.org/

Traditional HBM XML mapping

<?xml version="1.0" encoding="utf-8" ?>  
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"  
  namespace="QuickStart" assembly="QuickStart">  
  
  <class name="Cat" table="Cat">  
    <id name="Id">  
      <generator class="identity" />  
    </id>  
  
    <property name="Name">  
      <column name="Name" length="16" not-null="true" />  
    </property>  
    <property name="Sex" />  
    <many-to-one name="Mate" />  
    <bag name="Kittens">  
      <key column="mother_id"/>  
        <one-to-many class="Cat"/>  
      </bag>  
  </class>  
</hibernate-mapping>

Fluent NHibernate equivalent

public class CatMap : ClassMap<Cat>  
{  
  public CatMap()  
  {  
    Id(x => x.Id);  
    Map(x => x.Name)  
      .WithLengthOf(16)  
      .Not.Nullable();  
    Map(x => x.Sex);  
    References(x => x.Mate);  
    HasMany(x => x.Kittens);  
  }  
}