There are three ways using jquery to check if checkbox is checked. Here they are:

// First way 
$('#checkBox').attr('checked'); 

// Second way 
$('#edit-checkbox-id').is(':checked'); 

// Third way for jQuery 1.2
$("input[@type=checkbox][@checked]").each( 
    function() { 
       // Insert code here 
    } 
);
// Third way == UPDATE jQuery 1.3
$("input[type=checkbox][checked]").each( 
    function() { 
       // Insert code here 
    } 
);

First two methods return true or false.
True if that particular checkbox was checked or false if it is not checked.

The third method iterates though all checkboxes with checked attribute defined and performs some action.

I tried using third method on jQuery 1.3, but seems not work for me.