I have taken apart 12" powerbooks several times, its not THAT hard, you just have to be careful, and patient.
Important to lay out the screws and not mix them up, there are a LOT of those tiny screws. This is critical, there are over 30 screws you have to remove, and 3 or 4 sizes, make sure you dont mix them up. I would say the single biggest headache when I work on a PB12 is just keeping track of what screw goes where.
When you unsnap the top cover off the case, (the large metal top thats directly under the keyboard and includes the trackpad) make sure not to lift the rear too much before you lift the front. Lift the rear part just enough to get it out of its snaps, then move to the front. If you angle the top cover up too far at the back, the front clips will be jammed, and much harder to release, you could even bend the aluminum case they will be so tight. Keep the top as level as you can as you work around to unsnap it.
To replace the optical drive, you have to gut the PB12 and remove the motherboard. This is because 2 of the optical mount screws are under the motherboard.
The heatsink on the 12" uses thermal paste on the cpu, and thermal pads on the chipset, and gpu. Be very careful lifting off the heatsink, lift the cpu side first, then slide it diagonal upwards away from the gpu so you dont tear the gpu thermal pad. Another thing about the heatsink to be careful, is the 2 spring loaded screws that press it down on top of the g4 cpu. They thread into pegs that are bonded to the motherboard only. One of my PB12 had the pegs torn out of the motherboard from somebody that probably made them too tight. Make sure to press down to relieve the spring pressure when you remove them, and when replacing, press down, screw then down until they bottom out, then release to allow the spring to hold the heatsink. Dont torque them tight after they bottom out, just make them 'snug'.
Use the iFixit guide for powerbook 12" 1-1.5ghz, it is excellent, and follow it to the letter one step at a time.
Make sure you have a spudger tool, if you dont, make one before you start. For all the tiny cable plugs in the PB12, never ever pull on the wires to get them out, use the spudger to pop them up, all the plugs have a small ledge area, you can pry against to lift it out.
Anyway, thats my 2cents, hope its helpful.
-Edited for clarity-
