If i execute an ADO.NET "delete" sentence in the database, How this affect the second level cache?
those rows deleted by the delete command, if they were in the second cache, are being deleted too?

I believe the SysCache implements the new Sql Invalidation logic which should do what you are asking. I haven't tooled with it yet to see though. I think it is only implemented in the trunk.

Generally speaking, NHibernate is oblivious of anything you do outside of it. Therefore, the second level cache is unaware of any raw ADO.NET operation carried out outside of the NHibernate API. As mnichols said, there is a cache provider that ties into SQL2005 (not SQL2000, I don't think) that enables NHibernate to be notified when something changed in the database. It may be the SysCache2 provider though. Once again, I am not too sure about that. If you are using any DBMS other than MS SQL2005, I believe you are out of luck.

Thanks. So if i want to, for example, delete some entries in a database using ADO.NET, how can i sync my database with the second level cache? If it's imposible is there any way to batch delete a lot of entries and mark as deleted those that remains in the second level cache?

If you can accept the second-level cache and the database being out of sync for a short time, you can set up the cache expiration policy to a relatively short time. You may have to set up different cache regions and attach different policies to different region. Alternatively, you can manually evict the deleted entries from the second-level cache whenever you delete something. See docs here:
http://www.hibernate.org/hib_docs/nhibe ... ssioncache

You can use ISessionFactory.Evict to evict entries from the second-level cache.