Ran into an (other) interesting Internet Explorer bug. Seems that if you have a form with only a single text input, hitting the enter button will not submit the form in IE.

<form action="" method="post">
<fieldset>
<label for="user_name">User Name</label>
<input type="text" name="user_name" id="user_name" />
</fieldset>
<fieldset class="button">
<button type="submit" name="submit" id="submit" title="Verify User Name">Verify User Name</button>
</fieldset>
</form>

The solution is to hide an additional disabled input for IE to find, using IE conditional Comments and hiding it from view with some CSS.

<form action="" method="post">
<fieldset>
<!--[if IE]><input type="text" style="display: none;" disabled="disabled" size="1" /><![endif]-->
<label for="user_name">User Name</label>
<input type="text" name="user_name" id="user_name" />
</fieldset>
<fieldset class="button">
<button type="submit" name="submit" id="submit" title="Verify User Name">Verify User Name</button>
</fieldset>
</form>