Thanks to all who replied!
I have narrowed things down quite a bit, and am pretty certain I have isolated the problem though I have no idea the mechanism behind it.
I can force the problem to recur, and I can eliminate it.
Aperture has nothing to do with the problem -- it happens in Preview (the OS image viewing program).
The problem with consistency is that when the problem occurs, I don't know on which images it will occur, or why, and until today I had no reliable way of knowing if I was seeing images with the lobsterization, or not. (I can see the lobsterization -- I can't tell whether it is a correctly rendered "bad" image or a badly rendered "correct" image.)
The color space is program-specific (I think) -- the problem is not program-specific.
No screen-proofing profile in Aperture changes the problem.
It is triggered by running a dual-monitor set-up from my MacBook Pro when the following conditions are met:
- the external monitor must be a wide gamut monitor, and
- that monitor must be the primary monitor, as set in Displays Preferences (in OS X, the location of the menubar determines which is the primary monitor in a dual monitor set-up).
IOW, when the primary monitor is a wide gamut monitor (on my set-up) I get lobsterization of many JPG files (as well as over-saturation of system colors).
If, running Aperture, I change to full screen and run the "Swap Displays" command, the problem instantly occurs. I surmise that the command does the same thing that moving the menubar in "System Preferences→Displays→Arrangement" does.
Can anyone try this out and see if it affects their system as well. I'm running 10.6.8. The lobsterization does not show in Quick View, but does show (for most Web JPGs) when those files are opened in Preview.
The workaround is only slightly awkward -- and infinitely better than not being able to rely on the screen colors.
One thing which I noticed which puzzles me: when I drag the Preview window from one monitor to the other, it flashes reddish or greenish then settles to the correct color. This is the change I get permanently when I run with the menubar on the external monitor.