[Amphiluke]

Примерно так:

<script type="text/javascript">
window.onload = function () {
    var sel = document.getElementById("D1"),
        txt = document.getElementById("T1"),
        vals = { "bla-bla": "бла-бла", "blu-blu": "блу-блу" };
    sel.onchange = function () {
        txt.value = vals[this.value] || "";
    };
};
</script>

...

<input type="text" name="T1" id="T1" value="<?php echo $this; ?>" />

<select size="1" name="D1" id="D1">
    <option value="bla-bla"<?php if ($var == 0){echo ' selected';} ?>>1</option>
    <option value="blu-blu"<?php if ($var == 1){echo ' selected';} ?>>2</option>
</select>