Line Height
<div id="parent">
  <div id="child">Content here</div>
#child {line-height: 200px;}

Vertical centering is achieved by giving the child div (the one containing the text) a line-height larger than the font-size. Some also set a height equal to the line-height on this div, but I haven't found it necessary.

This will work to vertically center a single line of text, but won't work for multiple lines of text.

Note: Additional styles not related to the above have been used to make this demo more presentable.