Let me introduce you to my new favorite form factor: The Embedded Compact or ECX. When I took this out of its box, even though I knew its dimensions, I was shocked at how small it was. I do want to caution that everything with these boards is proprietary including its cables and proprietary = expensive. So know what your getting yourself into. I've seen these available in either Atom or most excitingly Mobile Core 2 Duo configurations as this one is.
In terms of features you can see the PCIe x 4 slot, a DVI/VGA/HDMI out. However, there is actually an LVDS out as well inverter pins (yes the entire LCD conroller is embedded in this tiny motherboard). So you could connect the LVDS cable and inverter directly to this motherboard without any additional hardware. Now the question is can the native LVDS resolution drive the iMac's 20" screen? And does this solve the problem with the inverter for the 20" iMac - maybe?
The underside shows the RAM (4GB) in place (purchased separately) as well as a slot for a Compact Flash Card. This I also find to be a fantastic addition. I need all the space I can get and although I am not sure, I would be surprised is an embedded board was unable to boot from the compact flash memory. This could mean with a 64+gb card no need for a hard drive, no extra heat or power, and a much faster system.
Here is the 2.53 Ghz Core 2 Duo Mobile (Socket P Penryn Processor) and its very reasonably sized heat sink/fan.
This is the first board that I have felt makes an all-in-one doable. I have never liked the idea of shoehorning a mini itx or mac mini into the base at an angle. Plus I had extreme concerns about airflow. I imagine this sitting above the dvd drive with the cpu fan around the same place as the iMacs original fan. This board also allows incorporation of a dvd drive. Although not necessary anymore, I always felt it was part of the iMac's charm. In addition, if no other hardware is required that would leave the bottom for placement of a powerwave adapter (to use the original speakers) as well as extenders/ports to make the rear I/O look as original/professional as possible. It does use a power brick, but I am certainly ok with that as no internal powersource = less heat.
Now I could find this board impossible to work with, it may not fit at the top of the base, it may require a hard drive. I don't know, but keep this in mind the original iMac was an embedded compact motherboard, it only makes sense to use something along those lines.
I already know I can thread LVDS cables down the neck into the base with a replacement 20" LCD. However, I could also try to splice the existing panel to DVI. I'm not sure which way I'm going to go - but I am leaning towards LVDS (if the native resolution is compatible) as it is embedded onto the motherboard, thus will turn on/off with the motherboard etc. For people interested in doing this, you will need extra LVDS cable. The best place to get it is ebay. I find cables used in flat screen tvs are usually longer and of more use. Examples of LVDS cables from tvs are shown in the pic on the right.
The biggest problem I'm going to have is with that damn 20" inverter (I am not running high voltage cables through the neck again - too unsafe). Unlike the 15 and 17" I have not been able to get it to work independently. Although there are pins for the inverter on the motherboard, it is one short. I believe there is some voltage difference involving orange and yellow that turns the inverter on. I am hopeful that maybe by having an active dimmer hooked up that this may solve the problem. Obviously using the native inverter with the motherboard would be option number 1.
Option 2 would be to locate some other aftermarket inverter that will work. I have yet to find one that is "flat" enough to fit in the monitor casing, but I haven't looked for sometime.
Option 4 is using the 17". Although I'd prefer the 20", if getting the backlights to work on the 20" proves too difficult, I already know how to do so with the 17". And would be quite happy with a fully all-in-one modernized 17" iMac G4.
I'd love to hear advice from people who have some know how with led backlights etc. And as always I'd be happy to answer any questions.