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

Now the form will submit in IE when you hit the enter key!