I notice that that the checkboxes have been assigned a non-existent class as bolded below:

1. <span class="ui-icon ui-icon-ui-icon-checkbox-off ui-icon-checkbox-off"></span>

(this is also occurring in your demo here:  http://jquerymobile.com/demos/1.0a3/#docs/forms/forms-checkboxes.html)

cheers

Matt