The Willy Wonka abstract factory

I think for the time being I have said everything I have to say about outsourcing. So now I'll leave this controversial topic and start writing a bit about software patterns. So, if you are not so technically oriented, it will be boring for a while.
One software pattern I find extremely useful is the abstract factory. I think polimorphism is only really useful with abstract factories. However, there is a point where you must chose one (and only one) concrete factory. How do you choose?
One strategy I found useful is to use a map between a key and the constructor or the key and a structure containing the constructor. As you can see, all the constructors or structures are very similar, just like Oompa Loompas, and that's why I call this decission strategy the Willy Wonka.
The advantages is that only declarative code needs to be added to add new classes, so bugs can hardly be introduced