Fabio just made the announcement. You can download the source or the binaries here.
And don't let the alpha tag fool you... while we can never guarantee bug-free releases (who can?), 2.1 is very good already. We currently have 2 projects at work which have been using the NHibernate trunk instead of the 2.0 release for about 2 months now with no NHibernate-related problems 
Be sure to check the release notes to get an overview of new features, issues that were fixed, and some breaking changes.