Apache, PHP, and Missing POST Variables

The $_POST array is always empty after submitting a form.

I'm using the Zend Framework (although this problem doesn't appear to be related). I have an .htaccess in the root directory. It looks like this:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . index.php [L]

The URL /auth/login (path doesn't really exist. It's rewrite magic) shows a login form, which looks like this:

.. html ..
<form id="login_form" action="/auth/login/" method="post">
      <div>
            <label for="username">Username</label>
            <input type="text" id="username" name="username" value=""/>
      </div>

      <div>
            <label for="password">Password</label>
            <input type="password" id="password" name="password" value=""/>
      </div>
            <div id="formbutton">
            <input type="submit" name="login" value="Login" />
      </div>
</form>
.. html ..

Submitting the form ends up with an empty $_POST array. In the root directory's index.php (Where the rewrite rule is sending all requests), I've added to the top:

print_r($_POST)

And it's always Array().

Some interesting things that do work. If I change the form to this:

<form id="login_form" action="/test.php" method="post">

And in test.php I have print_r($_POST), I _do_ get the POST variables. If I create the physical directory /auth/login/index.php (with the same print_r), and change the form to this:

<form id="login_form" action="/auth/login/index.php" method="post">

I _do_ get the POST variables. However if I leave the form as this:

<form id="login_form" action="/auth/login/" method="post">

I _don't_ get the POST variables. In a nutshell, it seems like I only get the POST vars if I specify the name of a real file in the form action. However I can't specify a filename, because the path /auth/login doesn't really exist.