My wiki/forum/video research confirms what you found: The M2NPV-VM is likely the best supported mobo for linux MCE -- perhaps for both the core and media director. No surprise that it's discontinued, but it does pose a problem for those of us who are seeking a known-compatible mobo.
It would be great to find out from others what mobos and peripherals are known working, but so far I haven't had much luck getting concrete compatibility information except for the M2NPV-VM.
From what I have read the M2NPV-VM is recommended mainly because it is proven to work (it was used in the video). It's based around the nForce 430 MCP. It looks like the 430 is no longer in production so it's no surprise the M2NPV-VM is also out of production.
FWIW, I plan to select a new mobo (not an M2NPV-VM) for a LinuxMCE box probably in the next month or so. Here is what my criteria is so far:
- $150 or less
- AM2 socket
- onboard known-compatible nVidia GPU
- onboard known-compatible Gigabit Ethernet
- onboard known-compatible S/PDIF out
I didn't include HDMI-out in my criteria. It seems that going from DVI out on the PC to HDMI in is just a matter of buying the right DVI->HDMI cable. I haven't tried this myself, but most reports I have read indicate that this works just fine. Given that, I think HDMI out is safe to leave off the criteria list.
My plan of action to narrow down the mobo model was to nail down which nVidia MCPs and GPUs are working for people in the Ubuntu and MythTV forums, then select an appropriate motherboard using a known-working MCP and GPU.
If you do narrow your search at all, I'm definitely interested in hearing the results.