Other newer developments include smaller form factors. I have yet to complete an all-in-one - my 17" hides behind it a computer. It is possible to shoehorn a mini itx board at the right angle into the base or a caseless last generation mac mini. Despite this - I would be very hesitant to put anything hotter than an atom-ion combo in the base. The G4 was designed as a convection cooled computer. The airholes are located next to the exhaust fan and the mobo is otherwise surrounded by a heavy metallic faraday cage. So there is NO air movement at the bottom of the dome. However, some decent smaller form factor boards specifically the embedded compact have just come on the market. Although they use mobile processors - a core 2 duo is would still make for solid computer. Also, using the splice technique no monitor components will waste precious space in the dome.
The 20" still remains the toughest challenge. While it takes only a power source and a few resistors to turn on the backlights to the 15" and 17" mods - I have never been able to get the inverter to work unless connected to the original iMac. Because no aftermarket inverter is thin enough to fit inside the monitor casing - it has to go in the dome. As a result, there are 6 high voltage wires that need to traverse a moving metallic neck. It was because of this that I disassembled my original 20" mod. I did not believe I had adequate shielding on the wires. However, I have recently come up with a possible solution that I will be investigating in the next few weeks. Thanks for following along and as always feel free to ask any questions.