I think lot has to do with the ambient temp in your room.
My temps in the summer for the Mobo/Case are around 45-49, in the winter they are in the 30's. My rooms gets pretty hot in the summer and my system temps actually get higher than my CPU temps.
I think as long as your other temps (CPU/Graphics Card) stay cool (40's CPU and60-70's for GPU) you should be fine.
Also depending on where that sensor is. Mine is right near my GeForce 8800, which radiates a ton of heat (idles at 60), so the heat blowing off of that heats up the air around the sensor. I added a fan to circulate the air in that area better and in dropped my case temps by about 5 degrees.