I've been wanting to do this for a while, but finally I'm doing it.
Project 1: Grape trayload iMac 333.
Screen is capable of displaying video from alternate source without logic board connected or even installed. However it can only display resolutions that correspond with it's fixed horizontal sync, so 640x480@117, 800x600@95, or 1024x768@75. I think there are a few more, but it will never show the bios post screen on most mini-ITX motherboards. I'll be setting that up with a different screen before final assembly. Afterwards if I need to access the bios I will connect an external screen. Shouldn't be a problem, plus not seeing the bios on boot will make it seem more Mac like.
Power supply capable of powering low power mini-ITX motherboards. I have it running a Zotac ION-ITX-D-E and it works great. I made a custom wiring harness using the pinouts from a link listed in an article on here describing how to put iMac G3 motherboards in ATX cases, so I plug into the iMac's power filter board and then I have a 20 pin atx connector, one sata power connector, and a sata slim drive connector. I hooked up a 500GB seagate desktop drive, and a lite-on slim dvd burner and it powers all of it even though the iMac's power supply is only 80 watts.
The drives will fit in the chassis the same as the originals. The tough part is that I have to trim the part where the motherboard goes in order to get it to sit low enough. I think it will work out though. Then I have to decide how I want to do the video cable since the Zotac board doesn't have any internal video connector. It has an external VGA. One idea was to mount some sort of connector underneath the motherboard that will make contact with the solder pins from the vga connector, and then that way I won't have to have a cable running from the external connector to the iMac's internal video cable.
I've also done a lot of research concerning running OS X on the Zotac Ion boards, and it seems that they are decent. A lot of work, but once it's done, it looks good. The main concern now is that people have found that the 10.6.2 kernel doesn't run on Intel Atom CPUs, so you have to go back to the 10.6 kernel until someone makes a custom 10.6.2 version. Not a big deal.
So, it looks like that one will work out just fine!
This involves doing this same mod with a slot loading iMac G3. I have a few of them to play with, but they are more complicated. I made a few custom wiring harnesses for the video and such, but was only able to power on the power supply. I read in another forum somewhere that the screen on those will only display video if the logic board is drawing power, but I don't know if that means it sense whether power is being drawn from like the 5 volt supply on the down converter board, or if there is a special sense pin for that. The pinouts for the slot loaders are just as well documented, but for the most part only the pins you need for putting the iMac logic board in an ATX case are explained. There are a few labeled pins that they don't use for that mod, but I don't know what those pins are for. I'd really like to do this mod on one of these since they have a 150 watt power supply and could probably run a mini-ITX board with a 775 socket cpu, or even one of the new 1156 socket ones like a core i5 when they come out next year.
So basically I know project 1 will work, and if anyone has any insight on project 2 please leave a comment. I imagine I will try testing it with a draw of some sort hooked up to the down converter board.