Take a look at this too http://www.patrick-wied.at/static/heatmapjs/

This is a great idea, and I can well imagine using it.

But the Google Maps integration seems a bit shaky at present -- the overlay generally fails to refresh when I scroll the map (FF 4.0.1 on Mac).

I enhanced Sunng's googlemap page by adding google map event and event listener. Here you can see the live demo. http://nihuajie.com/heatmap/googlemap.html

There are some redraw issue on panning the map. It seems that GoogleMaps won't redraw unless the panning distance is large enough. I will try to smooth it in future release.

Very nice. How much data does it handle before it starts getting sluggish?

Since HeatCanvas is pixel based, it costs time to render. However, the calculation and rendering are processed in background, which won't freeze UI. Both calculation and rendering are in O(n) to data input. And the function you choose for v=f(d) also affectes on performance.