With transforms being more ubiquitously supported these days, you can do this without knowing the width/height of the popup
.popup {
position: fixed;
top: 50%;
left: 50%;
-webkit-transform: translate(-50%, -50%);
transform: translate(-50%, -50%);
}
Easy! JSFiddle here: http://jsfiddle.net/LgSZV/
Update: Check out https://css-tricks.com/centering-css-complete-guide/ for a fairly exhaustive guide on CSS centering. Adding it to this answer as it seems to get a lot of eyeballs.