Cybra.Net : Geekery Abounds

This is soley because I'll forget (again) and then I can look it up.

Set top to whatever, left to 50%
Then if the element is say, 200 px wide
Set the left margin to negative 100

Easy. If you actually remember the trick.

#logo {
position: absolute;
top: 0px;
left: 50%;
margin-left:-100px;
}