To center vertically a content, you have to use the following CSS code

div.vCenterWrapper
{
    height: 100%;
    display: table;
    overflow: hidden;
}
div.vCenterContainer
{
    display: table-cell;
    vertical-align: middle;
}
div.vCenterContent
{
}

and the following HTML code

<div class="vCenterWrapper">
    <div class="vCenterContainer">
        <div class="vCenterContent">
            ...
        </div>
    </div>
</div>

Unfortunately the CSS code doesn't work with Internet Explorer versions prior to the 8: you have to add the following conditional code

[if lt IE 8.]>
<style type="text/css">
    div.vCenterWrapper
    {
        position: relative;
    }
    div.vCenterContainer
    {
        position: absolute;
        top: 50%;
    }
    div.vCenterContent
    {
        position: relative;
        top: -50%;
    }
</style>
<![endif]

If you want to center horizontally too, the temptation would be to use the text centering:

<div class="vCenterWrapper" style="text-align:center;">
    <div class="vCenterContainer">
        <div class="vCenterContent">
            ...
        </div>
    </div>
</div>

Unfortunately this method works with all browsers, but not with Internet Explorer 6 and Internet Explorer 7.
The correct way to do it, to center for example a 500 pixels column, is to modify the styles:

div.vCenterWrapper
{
    height: 100%;
    width: 500px;
    display: table;
    overflow: hidden;
}
div.vCenterContainer
{
    display: table-cell;
    vertical-align: middle;
}
div.vCenterContent
{
    width: 500px;
    margin-left: auto;
    margin-right: auto;
    text-align:center;
}