I experienced this problem recently, and found the solution at Macintouch. This from an iPhoto reader report:
"After updating my iPhoto library (4100+ pictures) to iPhoto 4, iPhoto would crash every time I tried to open the program. Reinstalling the program did not help. I moved my existing library onto the desktop, started iPhoto, created a new library, quit the program, and then moved my pictures and albums from my desktop library into the new one. I also trashed the thumb32.data, thumb64.data, and thumbjpg.data files. iPhoto seems to be reading the files fine now, and I did not lose any albums or any film roll titles. "
