We just dealt with this on a new Dell R940. Dell first replaced one of the four CPU and then the motherboard. We are still testing to determine if it resolved it.
From discussions with VMware and Dell I took away that a MCE can be caused by a bug in BIOS/firmware/drivers or due to a hardware failure. I'd recommend checking your systems against the HCL to ensure you are running the correct combination of firmware/drivers (In some cases we've had to downgrade firmware to align with the HCL).