If you're running against the trunk from Subversion, you probably need an additional line in your configuration to set up the ProxyFactoryFactory:
return Fluently.Configure()
.Database(MsSqlConfiguration.MsSql2005.ConnectionString(s => s.FromConnectionStringWithKey("my_connection_string"))
.ProxyFactoryFactory("NHibernate.ByteCode.Castle.ProxyFactoryFactory, NHibernate.ByteCode.Castle"))
.Mappings(m =>
m.FluentMappings.AddFromAssemblyOf<Program>())
.ExposeConfiguration(BuildSchema)
.BuildSessionFactory();
Check the InnerException property of your type initializer exception - that should give you some more detail about what's going wrong.
-D-