I'm always paranoid about screwing up originals of a file without thinking, particularly since I have a habit of pressing control-s a lot to make sure I don't actually lose any work (whether it's an image, text file, whatever). So I tend to use the Duplicate feature in the window menu of Paint Shop Pro. It copies the current file into a new window, and naturally asks me for a filename when I try to save it. I would assume Photoshop has a similar feature.
EDIT: Scratch that, I skimmed through the menus of CS2, and I don't see anything that does it, unless it's buried away. The only thing similar is an option to create basically a clone of the current window, and changes in one affect the other. I guess that's useful in its own way, but not for what I was referring to.