If you'd typed
echo $HTTP_POST_VARS['email'];at the top of signup.php, you'd have found that it didn't actually echo anything.
I'd change the code slightly if I were you. Rather than have a button for subscribe and one for un-subscribe, just have one submit button. So I enter my email address and hit submit, if I'm subscribed already then it un-subscribes me, if I'm not then it subscribes me. How does that sound?