Difference between quoted and un-quoted JavaScript object properties

Is there a difference between quoted and un-quoted JavaScript object property/method names?

For example, what is the difference between these two:

var obj1 = {
  property1 : "Value 1",
  method1 : function() {
    return true;
  }
};

var obj2 = {
  "property1" : "Value 1",
  "method1" : function() {
    return true;
  }
};