As the OP here, I'm a bit surprised to see the thread has come back to life. For myself, as I noted on 28th Dec 08, I've had no recurrence of the issue, which I put down to some weird one-off aberration that I'm unable to replicate.
In response to the recent posts: my thanks for your kindness in making suggestions; I do use ProPhoto in PS, and have been using it as my default working space for three or four years now. I have all the Color Management Policies checked to "Ask When Opening", and the "Advanced Controls" unchecked. Other than that, everything is set up on my machine much the same as Andrew Fee's, except I have my Gray space set to Gamma 2.2. If I change that to 1.8, CS3 gives me a warning as shown here (top left corner)
Since I only have PS & Bridge of the CS apps, I'm not sure what gives here, but I'm pretty clear this was not what caused the aberration that led me to open this thread...
Andrew's other suggestion - that I might have had soft proofing on - did make me wonder, but in my screen shot you can see "6)" in the bar at the top of the PS image window. And that's the end of the file name/ info string for a 16-bit image in standard (not soft-proofed) view. So not that one either... It remains a mystery, but is not a recurring problem.
So thanks for the suggestions, frugal, Wayne and Andrew, and hopefully someone will find some useful discussion and advice here in the future, but for me this topic is now concluded
Certainly proves that LL Forums are the place to post, though!
Thanks to you all