It does sound like a problem with the original PDF files the client sent you. You can check how they were made in Acrobat Pro from the Additional MetaData button on the Description tab of the document properties dialog. It will be under Advanced at the bottom of the list when the metadata opens.
You might be able to salvage them by running the optimzer and not changing the size, or just doing a Save As in Acrobat.
Thanks Peter, I've just done as you suggested and it's originally from Photoshop.
Yep the optimizing did the trick in Acrobat, but very strange as they'd worked previously. Maybe some clash with 10.6.8!
Try opening in Photoshop and resave there. If they have live type or other vector content resave as Photoshop PDF, but change the extension to PDP for placing in ID (which lets edit original work properly).
Thanks that's great!