To vertically center an absolutely positioned element with known height inside it's parent container is an easy task and guaranteed to work cross browser:
.centeredVertically {
position: absolute;
height: 16px;
top: 50%; /* push down by 50% of the height of the container */
margin-top: -8px; /* bring it back up by half of it's height */
}
Make sure you add position: relative
to your form so that it becomes the context for your submit button. See the fiddle: