From WebSphere Portal 6.1.0.1, users are using sametime chat through the web browser. After upgrading to Internet Explorer 8, the sametime chat window can be open but the chat functionality is not working. If a person pings you, a chat window opens but the screen remains empty.
To solve this, add in IE8 into the supported client in the portal administration. Go to Admin Page -- Portal Settings -- Supported Clients.
Initially, the lastest version is IE7. Thus make add a new client with IE 8. All the settings can be the same as the IE7 settings except the User Agent and Version.
After restarting the server, the sametime chat is working fine through IE8.