A couple of years ago, in an undergraduate seminar I taught called "Spam and Society," discussion veered a bit off topic. One of the students asserted confidently that airline Web sites give first-time users lower prices than returning customers. Most of the others immediately agreed. They said the motive was to suck in potential buyers; then, when they returned, the airline could quietly raise prices. I hear this kind of claim fairly often among heavy computer users. It seems to have become an article of faith that the unseen moguls behind all sorts of Web sites are cherry-picking consumers, customizing ads, manipulating prices and changing product offers based on what they've learned about individual users without the users' knowledge.