There are several ways to go about joining photos of the same location together.
The Photosynth team has plans to have neighbouring or overlapping synths be linked to each other in the future, allowing you to see them as one continuous environment, however no timeframe has been announced as to when we can expect this to happen. It is worth noting, however, that Bing Maps (whose division Photosynth is part of) has begun to really be active now, announcing automatic synthing of copyright-free geotagged flickr photos into Bing Maps Streetside several daays ago, and coming soon, Worldwide Telescope integration and live video tracking against the Streetside imagery as well. Hopefully synth linking isn't far behind.
For more on synth linking, see: http://getsatisfaction.com/livelabs/topics/linking_synths