Via the OSGeo-Discuss list, I learned about the first release of Acid Maps, an open source tool for real time heatmaps generation.
From the announcement: " Acid Maps is basically a web server that generates interpolated images from a set of valued points in real time. This means that you can use your existing datasets (e.g.: sales, temperature, etc...) to generate advanced visualizations: heatmaps, isolines and Voronoi diagrams.
In addition, custom "client" layers have been created for OpenLayers and OpenScales, thus allowing an easier way to communicate with the server."Read More »