I stripped down GOS's excellent how-to on using javascript to display a custom message instead of sending users to the unfriendly-looking Google Spreadsheet Forms confirmation page. The simplified code follows:

<script type="text/javascript">
function custommsg()
{
document.getElementById("form-message").style.display="";
document.getElementById("form-message").innerHTML="THANK YOU MESSAGE HERE";
}
</script>

<div class="ss-form-container" style="display:none;" id="form-message"> </div>

<div class="ss-form-container" id="form-container">
<form action="" method="POST" target="fake-target" onsubmit="this.action='http://spreadsheets.google.com/formResponse?key=YOUR-SPREADSHEET-ID-HERE'; custommsg(); return true;">

PASTE YOUR FORM CODE HERE

<input type="submit" class="ss-q-submit" value="Submit" />

</form></div>

<iframe src="#" id="fake-target" name="fake-target" style="width:0px; height:0px; border:0px;"></iframe>