A couple of days ago new version of Nhibernate was released. You can download new version here.
And if you migrate to this version and using Oracle database,
You will get this exception
Could not load type NHibernate.Dialect.OracleDialect. Possible cause: no assembly name specified
The reason is, you need specify not OracleDialect, but Oracle8iDialect, Oracle9iDialect, Oracle10gDialect, or OracleLiteDialect instead.
For example for oracle 10.x version you have to change dialect line in your hibernate.cfg.xml to this line
<property name="dialect">NHibernate.Dialect.Oracle10gDialect</property>