I would love to see better support for synths constructed from multiple parties' photos.
(For anyone who is thinking of doing this, let me just say that you should be tagging your photos' 'Author' or 'Photographer' field with the name of the person who took the photos before ever uploading them to Photosynth. That way their name will appear next to the copyright symbol as you flip through the synth. It's always interesting to see who took which shots.)
Back to my point, though. To my way of thinking, where multiple people's images appear within one synth, this should be notated in some way, on the right hand side of the synth. I would also love to be able to sort the photos in the viewer (possibly in grid mode although overhead view would be really excellent as well). Grid mode could easily sort different photogs into separate groups or associate each photographer with a separate colour. The colour per photographer concept could also apply very well in overhead.