First, I got to say that I am just starting out with Flex and Flash Builder so this may not be the most correct way to center your content.
Take a look at the Flex-In-A-Week training material, specifically Day 1, ex1_02_starter project. In that project a skin is used. In the skin Class they use <s:Group> with an id="contentGroup" and a horizontalCenter="0".
Hope this helps a little.