One might imagine that hand over the ear is pretty old and widespread, as many singers in India/Pakistan do it all the time, and I have read about it being done by ballad singers in this past century in the Balkans. I can say that in India it is definitely iconic of singers, especially of ballads and "weighty" material. And let's not forget that when you cover your ear, the other hand should be outstretched and raised!

For example, I did a quick search and found this (see especially after 5:20):
http://www.youtube.com/watch?v=D5sLONGdPjw