I have a friend that I help with her Mac issues from time to time and I noticed a few months back when she asked for help that I could no longer ‘Ask to Share User’s Screen’ in iChat.  As far I had ever seen, there was no where in iChat to turn on or turn off the Screen Sharing function.  Screen Sharing in iChat always required the user’s approval before a session could begin anyways, so it was always secure in that way and I believed the Apple did not have a control to enable or disable this feature, but I was wrong.
Tonight, while trying to screen share with her at home (I can get to her over VPN when she’s in the office), I was forced to troubleshoot this nuisance.  After a few minutes of Google research, I came across a post on Apple discussion forums that pointed me to the Video menu in iChat.  Turns out, there are controls there to enable Audio, Video and Screen Sharing or to disable them if you don’t wish to advertise these services to other users in iChat.  I have included a screen shot below to show the options.  Turns out that my friend had inadvertently disabled Screen Sharing at some point and with a quick click, we were back in business and I was able to see her screen and help with her problem.
