Basically, you store a mess of away messages. (Type in big box, add description, hit add button.) Then, you hit "Start Random Away Messages". Your away message will change every 20 seconds.
Probably not too useful, but I thought it was interesting that you could do that. (Although I see potential for abuse if people can set away messages arbitrarally from webpages.)
Update I haven't checked it extensively, but it seems to work in Internet Explorer, but not in Mozilla.