You mean that you want to use 120/240V and have a shield with the appropriate transformer on rather than have a transformer at the plug end correct?
Any specific reason for this...
It would be reasonably easy to make one but still not too safe. It would be easier to have an in-line transormer that is inbetween the arduino supply and the wall plug.
I am going to start working on one myself, but AC always kind of scared me.
Well it's probably not a good idea to have an exposed shield board with AC when you are building it if it scares you...
Mowcius