An article in CNET reports that there is a patent lawsuit brewing over challenge-response anti-spam software.

The good news is that there is a lot of prior art, and the folks who have it (including Brad Templeton, early chariman of the EFF] ) are very willing to help in the lawsuits.

Challenge-response anti-spam methods require the following of a URL or the performance of an action in order to let messages through (at least for the first time) and thus make it more difficult for messages not sent by a human to be passed on to your inbox.

EarthLink has announced that they will be employing a challenge-response system this May.