Having read 90% of the DAM Book and armed with just enough knowledge to be dangerous, I decided to play around with my old version of ACDSee (ver 5) and see if it could look at DNG files. I pulled up a folder with several DNG files and - lo and behold! - it showed the previews. However, they were all rotated sideways, so I picked one and tried to rotate it correctly. Well, the rotate function did some funny things and, though the picture was still there, I couldn't get it into the correct orientation.
Then I noticed the file size was now 158 Kb rather than the 4.6 Mb of the other DNG files! A quick check with Bridge showed that indeed the file had been written over and now was no longer readable by Photoshop. Doh!
Lesson learned: be very careful about using software not specifically designed for DNG files to catalog them.