no their is a third state: tristate or indeterminate i use it to make clear that the checked data is used for other sub checkbox you can see it in i.e if you use document.getElementById(checkboxid).indeterminate = true;
Does work for me in Firefox (version 3.6.4) and in IE, but not in Safari, Opera or Chrome. Is this even a standard? I have the feeling Microsoft invented this for the browsers.