could be good to see who is on line and have the option to chat with them. Alternatively have a counter to see how many people are on the Photosynth site at any one time.
Would you want the chat service to be part of the web site, or would you want a way to share LiveIDs so that you can chat on Live Messenger?
My wish would be : as you watch the synths of somebody, a simple click to send him a private mail
I'm going to take Fred's suggestion one step further and bring my hope for shared viewing back up.
I honestly think that building IM directly into the Photosynth viewer could take the activity of the site through the roof.