The Broadcom SoC doesn't have Ethernet built in, it's a USB connected Ethernet on the normal Pis. The USB bus from the SoC is brought out onto the SODIMM connector on the Compute Module so one could easily add Ethernet on the carrier board if needed. The Ethernet used on the normal Raspberry Pi 3 is a Microchip/SMSC LAN9514 USB hub/Ethernet part: http://www.microchip.com/wwwproducts/en/LAN9514

There are likely other methods of networking, if one wanted to avoid paying for a USB/Ethernet adapter chip for each Compute Module. Such as running a USB gadget on each Compute Module within Linux and have all the Compute Modules connected as USB devices to a USB SS host. If you used USB SuperSpeed hub chips and did the network switching on a non-Raspi SuperSpeed host within Linux (software defined networking!), this could likely easily get high-ish speed (ie: 100+ Mbps) networking working cost effectively, as the SuperSpeed hubs should allow upstream to run at USB SS rates while downstreams all run at USB HS rates, 7 port USB SS hubs aren't that expensive of chips. Microchip sell the USB5807 7-port USB SS hub chip for about $5, which is only slightly more than the LAN9514, as then you'd only need one hub for every 7 Compute Modules instead of one LAN9514 (or similar) per Compute Module.

http://www.microchip.com/wwwproducts/en/USB5807

Any comments on using Pi Zero as base line module?

  Supposedly only $5 per PCB from  
       https://www.adafruit.com/product/2885


  Power and network can go thru usb. (usbnet)