HiLoGenerator

In your Infrastructure project, under NHibernateMaps\Conventions:

public class PrimaryKeyConvention : IIdConvention
{
    public void Apply(FluentNHibernate.Conventions.Instances.IIdentityInstance instance)
    {
        instance.Column(instance.EntityType.Name + "Id");
        instance.UnsavedValue("0");
        instance.GeneratedBy.HiLo("1000");
    }
}

Create the following table:

CREATE TABLE [dbo].[hibernate_unique_key](
       [next_hi] [int] NOT NULL
        ) ON [PRIMARY]

Populate the column with a number to seed and you're done.