iBook G4 12" 800 MHz-1.2 GHz Hard Drive Replacement
I like to clone the old hard drive contents to the new one (in a FireWire or USB enclosure) with SuperDuper before I tear the computer apart then boot off of it to make sure I won't have any problems.
For those of you having issues with the computer not recognizing the new hard drive as bootable, make sure it's partitioned correctly when you erase it with Disk Utility. GUID Partition Table to start up an Intel-based Mac or Apple Partition Map to start up a PowerPC-based Mac like the iBook G4.
iBook G4 12" 800 MHz-1.2 GHz Lower Case Replacement
These 3 screws actually use 2mm Allen (hex) heads, so try to find one of those first (they come in many metric Allen sets) before resorting to the T8 Torx.