Digital cameras attach information to each photo you take.
This information - called Exif data - contains the time-stamp, aperture and shutter time amongst other things.
However some brands of cameras also attach a lot of additional, superfluous data.
This is inconvenient for a number of reasons, but most of all the fact that iPhoto 5 caches all this data in it's Library file resulting in slower operation after each new import of photos.
Affected camera brands include: Casio, Konica Minolta, Kyocera, Nikon and Pentax - amongst others.