Transforms rotate elements at their center point, so you need to place your <span>center in the center of the sidebar before transformations.
First of all, you do not need a top:50%wrapper div, you can do it all in between.
.
width:500px. , , 500 . , .
text-align:center. , .
position:relative, .
top:50% margin-top:-10px. 50% , , . 10px - .
left:50% margin-left:-250px, , . 250px - .
, , .
HTML:
<div class="tileStrip" style="position:relative;width:100%;height:185px;background:#DDDDDD;">
<div class ="clickToSlide" style="position:absolute;height:100%;width:30px;background:#AAAAAA">
<span style="font-weight:bold;color:white;display:block;text-align:center;width:500px;position:relative;top:50%;margin-top:-10px;left:50%;margin-left:-250px;-webkit-transform: rotate(-90deg);-moz-transform: rotate(-90deg);">International</span>
</div>
</div>
: http://jsfiddle.net/jHrEm/7/