Again! Is there a model to monetizing a custom Twitter app? whats the advantage of reusing twitter compared to building an independent twitter style app that does the job?
If it's a webapp, you can always use AdSense or equivalent, but Featured Users is a nice way incorporate your monetization into the overall Twitter ecosystem. The model is that people buy banner placement in Twitter apps to try to get more followers. The app developers call the Featured Users api to show an ad and then get paid based on clicks.
One Forty is trying to be the appstore for Twitter apps - that would include desktop apps as well as webapps so I would check them out as well.