An ongoing issue with PDFs lately revolve around certain PDFs showing grey placeholders instead of actual images. I have read all kinds of solutions to this, changing all kinds of settings. Some of those changes would work, some wouldn't. After it happened to me again, I thought I would try to find a different solution than what I had been doing (namely, changing the Internet options within Adobe Reader and/or Acrobat Pro).
A little poking around, and I found what seems to be the most viable solution yet:
EDIT > PREFERENCES > DOCUMENTS > PDF/A View Mode > Always.
The setting is about halfway down the "Documents" screen and is most likely currently set to "Never." Click the drop-down for that and change it to ALWAYS. After that, EVERY grey-boxed PDF I have opened has show the images! Wahoo!
UPDATE/EDIT (10/28/10) - Several folks have reported not having an "ALWAYS" option. I am checking into this and will hopefully have a solution soon!