<script language="javascript">
$(function(){
 
$.ui.autocomplete.prototype._renderItem = function (ul, item) {
$('.css-checkbox:checked').each(function() {
          chkId += $(this).val() + ",";
        });
        chkId =  chkId.slice(0,-1);
        alert(chkId);
 
 
    item.label = item.label.replace(new RegExp("(?![^&;]+;)(?!<[^<>]*)(" + $.ui.autocomplete.escapeRegex(this.term) + ")(?![^<>]*>)(?![^&;]+;)", "gi"), "<span style='font-weight:bold;color:red;text-transform:uppercase;'>$1</span>");
    return $("<li></li>")
            .data("item.autocomplete", item)
            .append("<a>" + item.label + "</a>")
            .appendTo(ul);
};
 
    $('#input1').autocomplete({
        source: "t1.php?cat="+chkId,
        minLength: 3
    });
 
 
});
</script>
</script>

<input type="checkbox" class="css-checkbox" name="category[]" value="1" />1

    <input type="checkbox" class="css-checkbox" name="category[]" value="2" />2

    <input type="checkbox" class="css-checkbox" name="category[]" value="3" />3

....
source: "t1.php?cat=здесб нужно передать значение ",
....