If you need something like this in a pre 4.3 version, here is a rough hack I was working on before I realized I didn't need this after all.

        preg_match('/Fields\[(.*)\]/', $raw_pdf, $field_data);
        echo "Fields: ", htmlentities($field_data[1]), "<HR>\n";
        $field_data = $field_data[1];
        $key_value = explode(">><<", $field_data);
        $key_value[0] = substr($key_value[0], 2);
        $key_value[count($key_value) - 1] = substr($key_value[count($key_value) - 1], 0, -2);
        while (list(, $kv) = each($key_value)){
                list($key, $value) = explode("/V", $kv);
                $key = substr($key, 2);
                if ($key[0] == '('){
                        $key = substr($key, 1, -1);
                }
                if ($value[0] == '/'){
                        $value = substr($value, 1);
                }
                if ($value[0] == '('){
                        $value = substr($value, 1, -1);
                }
                echo "Key: $key Value: $value<BR>\n";
        }

The $raw_pdf can either be the $HTTP_RAW_POST_DATA before you write a submitted PDF, or it can just be the raw PDF data you would read from a PDF using, say:
$raw_pdf = implode("", file("/path/to/your/fdf/whatever.pdf"));

You'll probably need to tweak this to do something more interesting than just printing the stuff.

Hmmmm.  Think they'll throw me in jail for this post?  Oh well.