Let's say you are a group of people super concerned about your anonymity and privacy. But you are working towards a common goal (like WikiLeaks, for example) so you have to communicate among yourselves. Of course you don't want anyone to know that you are communicating. If one person in the group gets caught, finding others should be almost impossible. How'd you do that?
One naive way is to use Twitter. Come up with an algorithm for determining a hash-tag for each day. I.e., hash tag for Jan 11, 2011 will be different from the hash tag for Jan 12, 2011. Everyone in the group knows this algorithm. When one person has to send a message to others, they will tweet it with any Twitter account, but they will include the hash-tag of the day. If I am part of the group, I can find the tweet. It doesn't matter which account the tweet is coming from, so anyone in the group can change their Twitter handle whenever they want. People can come and go, and nobody would know.
You know what I really like about this? (i) You don't have to log into Twitter to search. You can do a Twitter search in an incognito window in the morning and leave it open all day: you get all the messages from everyone, and no one knows you were receiving the messages. (ii) Twitter is a good tool for this kind of communication because it throws away (or at least doesn't show to the whole world) old tweets. Anyone who is trying to make sense of the pattern in the tweets can only dig for so many days in the past.