It depends.
How about a 1U rack-mounted box, with redundant PSUs, hosting around a hundred of Pi SODIMMs, with a USB-Ethernet chip for each slot, every X (7..24) ethernet lines connected to a switch chip with a gigabit uplink on the back of the enclosure.
Current RPi USB-Ethernet is slow because of the USB-HUB-and-Ethernet chip used. You won't have a USB hub in this configuration, and you can use a better USB-ETH chip.
Also you don't need a single, huge, expensive PCB. You can have multiple small PCBs side by side each one hosting just X (see above) cards.
Put a small switch next to each RPi DIMM slot on the board so you can power them off and replace-them online.
Need more local storage for each node ? - you can have a SD-Card slot next to each Pi slot - there are 46 GPIO available pins for those kind of extras (RTC, temperature, front panel leds & reset button for each RPi DIMM)
And Liz just let slip in the announcement comments that Broadcom might enable some extra functionality (ETH, SATA or other fast interconnect) on those extra pins.
PI-nsta-cluster.