We've gotten a slew of reports of broken AIM available messages, including the iTunes status. Unfortunately, this is a problem with the AIM servers. iChat, Pidgin, Miranda, and other clients are all seeing the same problem as a result. Hopefully it'll be resolved soon. In the meantime, while Adium won't report an error message if you set an available status, it appears that your contacts won't be able to see the message. When it's fixed, it'll be fixed for everyone.