gistfile1.js

function contains(obj, value) {
    return obj && obj.indexOf(value) != -1
}

var checked = contains(sd_o, sd[i].id) // must be false or null to disable
var formElm = new Element('input', {'type':    'checkbox'
                                   ,'name':    'sideDish[]'
                                   ,'id':      'sideDish_' + sd[i].id
                                   ,'value':   sd[i].id
                                   ,'checked': checked})