I have some rewrite rules to detect it and serve up a small GIF:

http://www.glitchwrks.com/images/general/hotlink.gif

Mostly I had issues with folks hotlinking some of my higher resolution images in their own site/blog/forum posts and claiming it was their work. I set up the rewrite rules after noticing a big spike in bandwidth utilization and seeing a bunch of requests in the logs. Referrer took me back to the offending pieces.

It was suggested to use some sort of shock image instead, now that would be antisocial!