Yes and no. I tried hooking my old quad core Core 2 Duo MB with the C6100 node. It did not boot. Maybe there is a way to do it by fixing the PSU.WOW! So you basically just cut away the metal between the rear I/O shield and the first expansion slot and it fit? That is a great idea. Scary thing is that I was looking at those pictures thinking: "I bet you could fit a mITX system in there somehow too."
OS x 10.8.2 installed successfully. Dell c6100 node, Antec 1200 case, Modxstream-pro 500W PSU (with modified 12pin cable), GeForce 9800 GT 1 Gig video (PCIe ribbon), QVS USB to 2.1 Stereo Audio, Samsung 840 Pro SSD, 1Gig Nic (only one working), 2x24" Hp LP2475w.
Xbench score 209, Geekbench ~12000 for multicore.
I also thought I could use some fans with the original heat sinks, but the heat coming off those CPUs require really powerful fans, which would turn the whole thing into 747. I used Supermicro heat sinks with whisper quiet fans (included) from newegg.com. Those are really big but mounted to the c6100 node fit into the 5.25 width.I'm hoping of doing the same thing, except I plan on using it a node for my home nas, I soldered the 12pin Female header from the imposer board and solderer that directly to the CPU + GPU power cables on my PSU (it was a Roswell so I don't really care). What did you end up doing for heat sinks, I think the default heat sinks are kind of out of the question as you would need to mount fans right on top of them to use them... Any ideas on a couple small QUIET heat sinks that wouldn't require switching the back plate on the CPU?