Mark,
I tried your suggestion several times and several different ways. Every picture opened every time.
OK, so that old bug in Photoshop/Mac OS X isn't the culprit.
I think I isolated the problem so that it can be duplicated. If you open a picture from preview and (as soon as Photoshop comes forward) go back to PM before the picture opens, it will not open in Photoshop most of the time.
Interesting. So you do something like quickly click back into a PM window or use Command-Tab to switch back as quickly as you can and the problem manifests itself whereas if you wait for the image to open it doesn't ever fail?
BTW, is there a way to keep Photoshop from coming forward when you edit from preview? I would really like to go through an edit and put several pictures into Photoshop while staying in preview mode.
It appears that it is possible to use the Apple-provided launching API to suppress the activation of the launched application. We would have to provide a preference to control this and it would be Mac-only since I know of no way to do the equivalent behavior on Windows.
Another way you could do this in the meantime is to use tags, color classes, or ratings to indicate to yourself that you want to edit a file. Then once you get back to the contact sheet, you can select those items that have your 'indicator' on them and then open them all up at once.
HTH,
-Kirk