Using PM 4.5.1 0403 on Windows XP SP2, I get the same behavior as initially reported above.
However, on Vista SP1 things seem to be working on individual jpg's. On dng's, if I work with a single image, Apply does not seem to work. However, if I use the left/right arrows to go through a group of selected images, then Apply works.
I'll see if using left/right arrows has an effect on XP, when I am at that machine later today.
Dan Dill