Have a look at Google Maps
This is more like it. A map I can really pan. Google really pushes for smooth interaction in the user interface. It looks like they break up the map into many small chunks. I'd guess they send enough area around the actual displayed map to let you start panning then send along more asynchronously as soon as you indicate which direction you want to move. It's not rocket science, it's just attention to good user interface design. And the resources to spend time getting it right. Kudos.

Since I happen to need a good map of San Francisco right now, here are a couple things to see in SF:
The Moscone West Convention Center, where GDC 2005 will be hosted in about a month. Can't wait.

Directions from the Convention Center to Ruby Skye night club - a route that I hope to follow at least once this year. Ruby Skye was a great night club the last couple years that I've gone to GDC.

The directions on Google's map also have a nice design. Clicking on the start or destination in the map or a turn descibed in the directions pops up a callout box. The callout contains a blow-up map so you can see that area in more detail and the callout is placed so that it points to the location on the full map. This is a little like the turn-by-turn directions on yahoo (also helpful) but lets the user choose the important detail they want to see. This is not as useful for printing, but is great for reading the route to understand it.

I realize this is still in beta, but it really raises the bar for interactive web based applications.