that sounds like fun.
Basicly to me you need to try an do what Flickr do (
clicky), Except for it being a map it can be anything else.
The way i would approach it is to lay out a massive tiled image, same as GoogleMaps etc.. then have it pre-caching to speed up the download.
that will give you a base or area for ppl to zoom in an out of. place all of that in a large div to give the effect of the page moving.
I know of the effect you are after, i have read up on it somewhere, but i cant remember where or how to start it.
I will have a scout about, but for the meantime you can have a look through these links and see if there is anything you can poach.
http://edevil.wordpress.com/2005/11/14/javascript-libraries-roundup/http://ajaxpatterns.org/Tutorial_Links