A new update to the Photosynth for iOS app is now available on the AppStore. This update includes support for iOS6 as well as the new iPhone5 screen, and addresses the black thumbnail issue many of you have reported among other bug fixes.
Once you update the app, you can fix a black thumbnail on a panorama stored on your device, simply by editing its thumbnail from the Properties screen. Panoramas with black thumbnails on photosynth.net have already been reprocessed to reset their thumbnails. If you would like to change the thumbnail of a remote panorama, simply delete it via the Photosynth website and re-upload it with your preferred thumbnail.
The Photosynth Team