Pues yo pensaba que no era posible, pero ahí va .... http://www.w3.org/Style/Examples/007/center.html
"CSS level 2 doesn't have a property for centering things vertically. There will probably be one in CSS level 3. But even in CSS2 you can center blocks vertically, by combining a few properties. The trick is to specify that the outer block is to be formatted as a table cell, because the contents of a table cell can be centered vertically.
The example below centers a paragraph inside a block that has a certain given height. A separate example shows a paragraph that is centered vertically in the browser window, because it is inside a block that is absolutely positioned and as tall as the window"
vertical-align: middle }
<P>This small paragraph...