Apple said in April that it would discontinue support for AIM in iChat for people using a me.com or mac.com ID on Macs running OS X Lion 10.7.1 or earlier. Today is the the day.
From the Apple blog post back in April:
iChat functionality including text messaging via AIM, video chat, and screen sharing will no longer be available when used with a mac.com or me.com ID on systems running 10.7.1 or earlier after June 30, 2014. Follow the instructions below to continue using AIM services with your me.com or mac.com ID.
Users on newer versions of OS X, including Yosemite, will not be effected.
Apple has posted instructions on its support website instructing users running OS X 10.7.1 or earlier how to upgrade to a newer version of OS X that fully supports AIM.
People using the third-party messaging app Adium, or those who can’t upgrade to 10.7.2, can make a new AIM ID and add it to iChat using the “Creating a new AIM ID” instructions in Apple’s support doc.