How to disable iChat's popup "You are not connected to the internet"?
You should probably use another app for screensharing and messaging.  If you choose to do this, I reccomend Adium for messaging and TeamViewer for screensharing.  I searched the contents of the whole iChat application, and I couldn't find anything, which leads me to believe that the pop up (I've had it too) is not actually part of the iChat application, but used by iChat.