I just played around with iPhoto trying to do this and I couldn't find a way. Personally I wouldn't want this to happen with iPhoto anyway since it saves any changes from the original as a whole new file, thus it would double hard drive space required for your pictures. You'd have to change them before importing them into iPhoto... Which is why I'm saving up for Aperture... make all the changes you want and it won't take up HD space 🙂