While Mecca is a proper noun, when referring to the city in Saudi Arabia, it is also used as a common noun, as in "Reno is a mecca for gamblers" or "the skiing mecca of the US is Jackson Hole." When a proper place name becomes a common noun, is it still properly captialized or does one use the lower case to distinguish it from the actual city?

Also, is this usage in any way offensive to Muslims?