NHibernate 4.1.1 is now released.
For a list of resolved issues, see the release notes: https://github.com/nhibernate/nhibernate-core/blob/4.1.1.GA/releasenotes.txt
Binaries are available on NuGet and SourceForge: https://sourceforge.net/projects/nhibernate/files/NHibernate/4.1.1.GA/ https://www.nuget.org/packages/NHibernate/4.1.1.4000
The [NH-3904] has been reverted in favor of [NH-2401]: users now required to explicitly specify custom user type via MappedAs method if they want to use IUserType/ICompositeUserType type parameters in Linq queries, or implement generators the way they take the types into account.