On the leopard server page it says this about screen sharing (though oddly worded) which leads me to think it's going to be a network only feature.
"iChat Server 2
iChat Server works with iChat AV 4 in Leopard to bring easy-to-use, secure multiuser chat, file transfers, audio and video conferencing, and screen sharing to workgroups, small businesses, and large corporations. iChat Server 2 adds instant messaging federation, allowing users to chat with users on other XMPP instant messaging systems such as Google Talk and vice versa. Features include new automatic buddy list generation options and single sign-on authentication using Kerberos. For extra security, there are new chat log archival facilities and support for secure server-to-server data encryption."