gistfile1.js

// Usage: $form.find('input[type="checkbox"]').shiftSelectable();
// replace input[type="checkbox"] with the selector to match your list of checkboxes

$.fn.shiftSelectable = function() {
    var lastChecked,
        $boxes = this;

    $boxes.click(function(evt) {
        if(!lastChecked) {
            lastChecked = this;
            return;
        }

        if(evt.shiftKey) {
            var start = $boxes.index(this),
                end = $boxes.index(lastChecked);
            $boxes.slice(Math.min(start, end), Math.max(start, end) + 1)
                .attr('checked', lastChecked.checked)
                .trigger('change');
        }

        lastChecked = this;
    });
};