array_add_html_safe_join.rb

class Array
  def html_safe_join(separator)
    acc = self.shift
    acc = acc.try(:html_safe) || acc
    self.inject(acc) do |a, elem|
      a << separator
      a << elem
    end
    acc
  end

  alias_method :join_to_string, :join
  alias_method :join, :html_safe_join
end

replacing Array#join.rb

# development environment (Rails 3.0.0.beta1)
a = ["a".html_safe, "b".html_safe]
b = a + ["c"]
puts a.join(" ".html_safe).html_safe?
# prints false
puts a.each_with_index.inject("".html_safe) { |a, elem| if elem[1] > 0; a << " ".html_safe; end; a << elem[0] }.html_safe?
# prints true
puts b.each_with_index.inject("".html_safe) { |a, elem| if elem[1] > 0; a << " ".html_safe; end; a << elem[0] }.html_safe?
# prints true!