In my iBook, there was no need to remove the screw under the ribbon cable. It's just a part of the optical drive.
If your iBook is like mine (700MHz) The yellow screw was attached from the bottom through the bottom case and had to be removed earlier in order to remove the bottom case. Also the the green screw pictured to the left was missing already removed.
Don't peel back the long foil shielding pictured top right! It should stay attached to the bottom shield that you are removing.
You might wanna use your spudger to pry up those back tabs to. You might break the bottom case just wiggling around waiting for it to pop loose.