This thread hopefully puts a little light on the problem from OSX/filessystem point of view.
When I am copying my iPhoto Library from a Snow Leopard MacBook to a Lion iMac, iPhoto has the popular exclamation mark problem with many pictures. On both MACs is the same version iPhoto 8.1.2 installed. The jpg files in question are present in the library on the failing Lion iMac and have obviously the same permissions as other pictures in the same directory/event which are working fine.
In order to find out what happened to the Library during copy (tested a copy via USB drive as well as a network rsync copy), I chmod -R 777, chown -R username.staff and did a xattr -cr against the Library. All without success. Onthe Lion iMac many ofthe pictures are OK with the thumbmail, but if you what to see the full picture an exclamation mark is displayed.
If I copy this non working Library back the Snow Leo MacBook, it fails as well on the same pictures.
One thing is noticable: as soon as iPhoto starts with the copied Library the Library seems to be reduced in size from about 31GB to about 24GB. I have not yet investigated at what point in time this happens and why.
Does anyone has an idea what may happened here? Any changes to the file system? What can an Application cause to fail on a directory tree which was created on a Snow Leopard?