Just wrote this function. It will center the browser window on the input coordinate. I've tested it in Mozilla Firefox, Explorer, and Safari (on both PC and Mac where applicable).
xpos = xx - document.body.clientWidth/2
ypos = yy - document.body.clientHeight/2
I am still having one problem, however, in Safari. When I try to access this function using onLoad, it doesn't work at all. Here's my body tag: