I have a simple form at http://jsfiddle.net/np8ty/ and can't see any reason for the gap separating the red and beige boxes.
<form action="" method="POST" id="checkout_form">
<input type="text" size="20" class="cc-number" placeholder="Card number"/>
<input type="text" class="cc-exp" placeholder="MM/YY" />
<input type="text" class="cc-cvc" placeholder="CVC" />
<button type="submit" class="submit">Submit</button>
</form>