Constructing Ruby Class Instances from a String

OK, little bit of Rubyfoo here:

class Person
def say_hello
 puts "hello"
end
end

class_name = "Person"
person_class = Object.const_get(class_name)
person = person_class.new
person.say_hello

I should spend some more time in the Pickaxe book to try to pick up some more things like this...