I hope they fix screen sharing in iChat. Twice I have tried to use it with a friend to help them fix a problem and it has failed. It doesn't provide any useful error messages as to why, either; just "failed to connect".
It's likely a NAT issue. We hit this at home at our house. Not all apps can work when both endpoints are behind NAT. Since iChat uses standards-based protocols, Apple has limited choices in how they can try and rectify this issue.
When the connection fails, there is very little iChat can do to tell you why. The packets it expects are behind blocked by a router (usually), and it has no means of knowing why.
What they could do is put a help file in there explaining why it might be failing, but they cannot tell you decisively why.