The top and side bars are (more or less) simply HTML. The center pane with the map, however, is a different beast. First, let's address the map itself. It is broken up into a grid of 128x128 images (basically like an old tile-based scrolling console game). The dragging code is nothing new, but the cool trick here is that each of these images is absolutely positioned -- and the 'infinite' scrolling effect is achieved by picking up tiles that are off-screen on one end and placing them down on the other end. The effect is kind of like laying track for a train by picking up track from behind it.Link (via JWZ)
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. Personally, I didn't even realize you could depend upon the browser to render these correctly, but apparently (at least with IE6 and Mozilla), you can. And they actually render pretty quickly -- for proof, check out the overlaid route image (at the end of the article), which is often as big as the entire map view.