OK, let's say I need a UserType to be used as identifier...
Should I implement IUserType? That is obviously not quite enough since CustomType doesn't implement IIdentifierType... What is the recommended way to make a custom identifier type?

Thanks,
Alex

For identifier types you need to implement IType interface (and IIdentifierType) instead of IUserType. You can derive your class from one of NHibernate's internal types (such as AbstractType or ImmutableType) to make it easier.

Implementing IIdentifierType on a user type will not work. IUserType is wrapped in an instance of CustomType at runtime and CustomType does not implement IIdentifierType.