Okay, I just thought I'd chime in and try to help some people from going through the frustration that I recently went through with iChat in Leopard. Here's my story.
iChat screen sharing used to work for me and suddenly one day it stopped working correctly... lemme explain. I would let someone share my screen, and it would start working and they would wave their mouse around and 10 seconds later, it will freeze and lose connection with a timeout.
At first I thought it was a firewall issue but, after thinking about it.... I thought, how could it be if it was working for 10 seconds and suddenly stopped? If it was a firewall issue, it wouldn't work at all. Then I thought maybe my iChat app got corrupted, so I used Time Machine (yay!) to restore iChat from one of the first days I got Leopard. That didn't work. So then I figured my preferences got corrupt, so I deleted my com.apple.ichat.plist file and still didn't work. I was so mad that Apple made this feature basically useless for me until I had a crazy hunch.
After an hour if pulling out my hair, I finally had an idea of what might be causing the crash... I noticed I had my old iSight plugged in (the external firewire iSight) into my Apple Cinema Display. I unplugged it and iChat screen sharing worked PERFECTLY. You should also know that I don't have problems with my iSight, it works well on Video Chat and Audio Chat and even presentation mode but, it screwed up screen sharing.
In conclusion, if you're experiencing sudden connection losses with iChat screen sharing, check to see if you have an iSight plugged into your Mac and unplug it.
Also, you're welcome.