fwiw, I don't consider iphoto "safe" to hold more than about 5,000 photos. I recommend users switch to aperture, which besides having a lot more useful features, has vaults which are good for offloading pics you're not working with atm. (useful for projects like if you are a wedding photographer)
I've seen iphoto tank on too many occasions when stuffed with 7000+ pictures.
I work for the Department of Redundancy Department
