Fixing the copper piece from the corner on the back of the motherboard worked flawlessly with my 3gs! I was sure it was the LCD that was faulty at first...
I took the motherboard together with the dock and LCD out of the case, connected them, and it booted perfectly when I connected the phone with iTunes, but the white screen come back as soon as I put the phone back together in its case. I found this post later, and tried to lift and reposition the little copper frame, so that it will touch the back on the motherboard (just beneath connector number 1).
Thank you very much for the help guys, and good luck for those who are encountering the same issues with the screen.