<form ...>
...
<input type="text" value="" readonly="true" name="testo">

<select ... onchange="this.form.testo.value = this.options[this.selectedIndex].value;">
<option value="uno">1</option>
<option value="due">2</option>
<option value="tre">3</option>
...
</select>
</form>

E` questo che volevi?

Ciao
Michele