brand.rb

def self.resolve(brandish)
  case brandish
  when Brand
    brandish
  when Fixnum
    find(brandish)
  end
end

product.rb

named_scope :by_brand, lambda { |brands|
  brand_list = [*brands].map{|b| Brand.resolve(b) }
  {
    :include => [:brand, :classification],
    :conditions => ["brand_id IN (?)", brand_list.map(&:id)],
    :order => 'classifications.name'
  }
}