When it comes to absolute positioning, you are stuck aligning everything to the left or right of the browser. There is no way to align from a center axis.
I was faced with that a few days back, and it turns out it’s entirely possible, actually, given a bit of monkey work. Here’s my solution, which holds up in everything I’ve checked it in so far.
It strikes me that there’s a simpler way to do it, but the example I’m working with hasn’t duplicated when simplified. I’ll update if I manage to get it.
update: IE5-friendly example added.