Apple released iPhoto 4.0.1 today in your Software Update:
iPhoto 4.0.1 includes many organizational and stability enhancements. Performance has also been increased throughout the application, giving you faster importing, smoother image viewing and easier Rendezvous photo sharing. This version of iPhoto also features improved thumbnail images in your photo library. To take advantage of this, your thumbnails must be upgraded, which may take a while, depending on the number of photos in your library.
iPhoto 4 was originally released with iLife '04 in January.