[monolithed]

оно?

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>

<script>
$(function() {
    var form = $('[name="form"]'), 
        array = ['Челябинск', 'Севастополь'];
    $('select', form).change(function() {
        $('input', form).removeAttr('disabled').val(array[this.selectedIndex-1]);
    });     
});
</script>

<form name="form">
    <select>
        <option value="0">-- выберите --</option>
        <option value="1">Россия</option>
        <option value="2">Украина</option>
    </select>
    <input type="text" value="" disabled="" />
</form>