Stumbled across this bit of interesting information. Two interesting points - first, it's possible to embed thermal sensors onto a memory DIMM:
“In order for the Mac Pro to maintain a safe operating temperature of the FB-DIMMs, the AMBs for each FB-DIMM must provide a functional temperature sensor.”
This is thanks to the SPD (Serial Presence Detect) chip that's on every DIMM that also supports the I2C protocol.
Second, the Mac Pro will spin up it's fans to maximum rev if it can't find the sensor:
“The Mac Pro will run the fans at full speed when detecting FB-DIMM's without a functional temperature sensor, which will maximize cooling on the FB-DIMM's.”