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> 
 
     
     
    