As I wrote the guide and the temperature monitor, I might answer as well :)
The specifications for these drives say that the operation temperature is 0-60°C. If your drives are constantly running hotter than 55°C, it might not be healthy for them.
My MBWE is located in a closed cabinet with my dsl modem and a gigabit router. The idle temperatures (drives spun down, fan not spinning) for the drives are ~38C during night and ~43C during day (much more traffic through the router and the modem -> the cabinet gets warmer). When the drives are spinning and in use, the temperatures almost never rise above 50C. The drive closer to the mainboard runs always a few degrees hotter than the other, probably because the cpu is warming it.
I have replaced the original 40mm fan with two quiet 120mm fans, one blowing air into the MBWE from the top and the other sucking from the bottom. I have also removed the metallic cover from the top of the drives in order to improve the airflow. (the modification is ugly as hell, but it doesn't matter since my mutilated MBWE is hidden in a cabinet and it seems to be working and completely silent :)
You can set the speed of the fan with
echo [number] > /sys/devices/platform/wdc-fan/speed
Where [number] is the desired speed. I haven't tried other speeds than 0, 50 and 100 so I have no clue whether they will work.