var query_string = '';

$("someElement").each( function() {
    if (this.checked) {
        query_string += "&test[]=" + this.value;
    }
});

$.ajax({
    type: 'POST',
    url: 'test.php',
    data: 'id=1' + query_string,
    success: function(t) {
        $('div#content').empty().append(t);
    },
    error: function() {
        $('div#content').append('An error occured during processing');
    }
});