Darin, this is simple enough to do with javascript (with the obvious caveat that it won’t work in non-javascript browsers and if the user has disabled javascript intentionally). all you need is an “onsubmit” part in your form’s opening tag. here’s a small example:
<form name="testform" method="post" action="process_form.php" onsubmit="this.elements['submit'].disabled=true;">
<input type="submit" name="submit" value="send this off...">
</form>