letterwoman-
To copy FFOs from one frame to another with the Eyedropper, you must have Fill:Color and Tint selected in the Eyedropper options. Double click the Eyedropper in the Tools panel to see the options. A little weird, IMO. What do Color and Tint have to do with Crop, etc? Not much.
Anyway, what this means is if you first click on a placed graphic, the Eyedropper will copy the fill color of the pixel you click on instead of the FFOs. So you have to click in an empty area within the frame to copy FFOs. Obviously that's no problem if the frame is empty, but if it's filled with a graphic, you'll have to either resize the graphic or the frame to make some empty space to click on.
Unfortunately, if you resize the frame, you change the crop offsets. Depending on what you're trying to accomplish this may or may not matter. ie, if you're trying to get art to be a specific distance from the top left of the frame, you may not really care about the bottom crop, so you could open up the bottom of the frame to use the Eyedropper.
It's all a little sketchy, but CS5 is still a huge improvement over CS 3 and 4 where there was no good way of pasting a graphic between frames.