At the mo my motherbaords at 30 degrees C. There is nothing wrong with the motherboard being a bit hotter. I guess this being hotter would be because of other components heating it up (maybe where the temperature checking diode is), maybe athe PSU, Video card for example.
If the motherboard got to about 50 degree's I'd be worried. Anything between 40-49 i'd be rather worried. Below that should be fine
Hope this helps