Found this great article that explains how Google Maps works.  It seems to be an original combination of DHTML, CSS, JavaScript, XML, and XSLT.  The built in XSLT engine transforms the XML to HTML which also explains why Google Maps only works on IE and Mozilla, it appears that this was a Microsoft technology that was later copied by Mozilla.  The others such as Safari and Opera lack a built-in XSLT processor.

In the future it may look like Google will open up the API or at least allow XML packets to be sent from the Google Maps site.  As Jon Udell points out you use to be able to append the term "output=xml" to any Google Maps URL, the server then sent back an XML packet, however since Udell's writing of this the feature has been disabled.

The article can be found at http://jgwebber.blogspot.com/2005/02/mapping-google.html.