order_by_rand.rb

# http://en.wikibooks.org/wiki/SQL_dialects_reference/Functions_and_expressions/Math_functions/Numeric_functions
def rand_function
  @rand_function ||= (
    if [ "SQLite", "PostgreSQL" ].include? ActiveRecord::Base.connection.adapter_name then
      "random"
    else
      "rand"
    end
  )
end

# example
Model.find(:first, :order => "#{rand_function}()", :limit => 1)