Dynamically Picking High Contrast Colors?

FocaJonathan asks: "We are creating a Web-based app that pulls down color attributes from a user specified Web page and builds a custom page for users trying to maintain the color scheme from the specified page. Most of the time we can simply grab stuff from the body tag and look for tables and font colors used. However, periodically somebody does something wacky and we end up with the same color for a foreground and background color. When this happens, we would like to automatically select a pleasing, high contrast color given the one we already have. As dynamic page generation grows this will be a useful question, but I'm sure somebody must have explored this for map highlighting, dynamic copyright additions, or some visual design project. Any ideas out there?" The problem many people have been struggling with for years, though: How do you technically define "pleasing"?