Alinhamento Vertical dentro de uma div

< ?xml version="1.0" encoding="iso-8859-1"? >
< !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" >
< html xmlns="http://www.w3.org/1999/xhtml" xml:lang="pt" lang="pt" >
< head >
< title >Vertical Align< /title >
< style type="text/css" >
< !--
#main {
display: table;
}
#row {
display: table-row;
}
#cell {
float: none;
display: table-cell;
vertical-align: middle;
background-color: #CCCC00;
height: 200px;

}
-- >
< /style >
< /head >

< body >
< div id="main" >
  < div id="rows" >
         < div id="cell" >ok< /div >
  < /div >
< /div >
< /body >
< /html >

Fonte: http://forum.imasters.com.br/topic/132304-tableless-alinhamento-vertical-o-mito/