var s = document.getElementById('field[12]');
var cust_cc_exp_month = s.options[s.selectedIndex].value;

// This displays the correct Value -- which is "0" for item #0 = "Please select..."
alert("Month: " + cust_cc_exp_month.toString());
if (cust_cc_exp_month == "0") {
  // This code never executes!
}

Here is the HTML portion:

<select id="field[12]" name="field[12]">
<option value="0">MONTH</option>
<option value="1" <?php if ($field[12] == "1") echo "SELECTED" ?>>January</option>
<option value="2" <?php if ($field[12] == "2") echo "SELECTED" ?>>February</option>
<option value="3" <?php if ($field[12] == "3") echo "SELECTED" ?>>March</option>
</select>