Photoshop About Image

What is the image format used to create the CS About screen with the feathers extending beyond the image 'boundary' and the drop shadow?

Well, it's hard to say considering the about screen is part of the code of Photoshop as well as being displayed by Photosop, ... it could be anything they want it to be including their own custom coding.

Otherwise it could be PNG format because PNG is the only format I'm aware of that allows for varying degrees of transparency/translucency, and that's the only way you can get a drop shadow that works over any background.

You can do the same with bitmaps. Its all up the the GUI API, whether it supports transparency.