I restarted...solved the problem. Started Photosynth again and haven't had any problems. What could have caused it? I hadn't restarted my phone for weeks before tonight. Maybe a memory issue?
Photosynth mentioned back in January that they were trying to track down some problems with camera drivers on some important models.
Sounds like they didn't quite get all of them, but I'm glad the app's out.
Happily, the problem seems temporary enough.
I can't seem to get a straight answer from anyone who's experiencing this error whether rebooting the phone once avoids the error from returning after you open and close Photosynth again, though.