You have a nice solutions as well, however I prefer to have an inner class which sole purpose is to restrict constructor access to private as I find this method to be an elegant solution as it allows an abstraction of singleton creation from Singleton implementation.

A Singleton class in my opinion should not contain any members (static or instance) which are responsible for creating the singleton object other than the static instance member.

Great post you have got there. Thank you! I am learning flex and actionscript3 currently, and I was a bit surprised to know, I could not mark constructors as private.

There is another option though, that avoids the need to create an extra internal class. You can simply put a static boolean flag inside the singleton, and let the constructor fail, if the flag is false. The static getInstance() method can then set and reset this flag on instance creation.

I have shown how here, in my post on ActionScript3 Singleton.