If the ibook G4 only gives a black screen and fan running full speed when powered on the problem is often related to the U28 chip issue (desoldered pins) on the logic board. The chip has 28 pins and is located near the DC in power socket under the logic board. You need to solder back two small desoldered pins (#1 & #28) to their contact point on the logic board. You need to use a pencil thin soldering iron like the Weller WM120 to avoid bridging the other pins:
http://www.amazon.com/Weller-WM120-120v-Pencil-Soldering/dp/B0000WT586
U28 chip location:
http://www.flickr.com/photos/metasim/532371992/in/set-72157600316722784/
http://www.macrevitalise.co.uk/overview%5B1%5D.jpg
http://fernieville.files.wordpress.com/2008/06/moz-screenshot1.jpg