Without using javascript (ala something like thickbox, etc. for positioning photos/captions centered), the closest I could come to was this:
Note that I had to specify some sort of height for the container (in this case the BODY but it could also have been applied to the wrapper DIV I think). In Explorer 6, you can set the BODY height to 100% but in Firefox this doesn't work and probably won't work in other modern browsers.
EDIT:
Found a better solution: