Here's a rather unconventional way of doing it:
#inputcontainer
{
display:table-cell; //display like a <td> to make the vertical-align work
text-align:center; //centre the input horizontally
vertical-align:middle; //centre the input vertically
width:200px; //widen the div for demo purposes
height:200px; //make the div taller for demo purposes
background:green; //change the background of the div for demo purposes
}
It might not be the best way to do it, and margin? won't work, but it does the job. If you need to use margin, then you could wrap the div that displays as a table cell in another 'normal' div.
JSFiddle: http://jsfiddle.net/qw4QW/