one of the (two, in my g4) HD bracket screws was stripped when the original HD was installed. the screw was soft, and the X was nearly an O. I drilled down the screw head with a dremel and was able to lift off the bracket. The headless screw unscrewed with pliers -- this left the thread intact though the screw wasn't usable. I had leftovers from a complete MacBook screw set which i'd gotten from iFixit for another machine. And one of them fit well enough.
On this site and others, I found a couple of other methods for deaing with stripped screws:
1. glue another screw head-to-head to the stripped one, using J-B Weld, then use plier to unscrew both. I didn't want to wait 6 hours for the glue to set so I didn't try this.
2. using a cutter wheel on a dremel, cut a straight ridge into the screw head and use a regular screwdriver to undo the screw. I tried this, but because the screw is embedded flush to the surface, I would have had to cut a ridge into the bracket as well.
3. if the screw were not aluminum (=so soft) i would have tried using very heavy downward force when unscrewing, to get the screw even slightly loosened, then finishing with pliers and an exacto or swiss army knife.
in my g4 there is also a big transparent ribbon (for keyboard?), to the right of the amber trackpad ribbon, that i disconnected before removing case cover.