How Google Maps works: a look behind the Javascript of Google Maps.
Whereas GMail uses XMLHttp to make calls back to the server, Google Maps uses a hidden IFrame. The method has its benefits.
The push-pins and info-popups are a different matter. Simply placing them is no big trick; an absolutely-positioned transparent GIF does the trick nicely. The shadows, however, are a different matter. They are PNGs with 8-bit alpha channels.
The comments following the post as also quite a read.