I'm not sure. When I updated my iBook today, I left it while it was downloading and came back to it a few hours later. The screen was off, however it wasn't sleeping. I assumed it had shut itself down, so I held down the on button, but then I heard it turn itself off. I turned it on and my LCD was all shimmery and it took AGES to get past the grey screen. Now after a reboot or two it seems back to normal, although my wallpaper image does look slightly shimmery.