Kunal Anand who works as a software engineer at NASA's Jet Propulsion Laboratory wrote a Python script to help him visualize his del.icio.us tag intersections.
As he describes in his blog: "The purpose for doing this was to help me see if I was even using multiple tags for my links. As it turns out, I have about 70 intersecting nodes (tags), 20 that are isolated (...) What is neat is that I have approximately 200 edges (intersections). The point? As predicted, I am not much of a combination tagger - instead, I am old-fashioned and use a tag to represent a folder". Each dot represents a tag (node) and each line represents an intersection between tags. The center of the visualization (denoted by a gradient), represents the heavy set of intersections.
Kunal says that he's also interested in comparing his method with other people's usage of del.icio.us. To do that all you have to do is e-mail him an XML of your links and he will run it through his script.