A new site called TweetStalk is in private beta. It allows you to “follow” Twitter users without them knowing you are doing it (Twitter tells you when someone new has subscribed to your data). It’s all through a Firefox Add-On and appears to modify the Twitter page itself via Greasemonkey or otherwise. You are then able to follow the person without them knowing, and the service provides a RSS feed as well.
This isn’t as bad as it sounds. Twitter pages are public by default so all the content is there for everyone to see anyway. Twitter should probably just implement a private follow feature of some sort to allow this anyway. But until they do, you’ve got TweetStalk.
Update: Nashville, TN based Sitening checked in to say they’re the ones behind the service. Find out more about them on Crunchbase.