I know a lot of people including myself have been having problems with iChat, problems like not being able to screen share, video chat or voice chat. I simple fix that worked for me was just turning off all the sharing prefs (surprisingly), than restarting iChat and trying again (if it than works you can turn your prefs back on), now if that doesn't work someone made a nifty helpful youtube video--->http://www.youtube.com/watch?v=pWtii7gHgeo. Hope this helps anyone that has been having any problems.