If you are having problems getting iPhoto 4.0.1 to update your thumbnails, check the permissions on your Pictures and iPhoto Library folders.
In my case, both folders had a group of “unknown” instead of “staff” Changing that allowed me to update my photo library!
And no, using Disk Utility’s Repair Permissions did not fix this, nor did Disk Warrior, nor any of the other of myriad of fixes.
I am still not able to rebuild my photo library by holding down shift-option while launching iPhoto. the rebuild gets to the end and then iPhoto crashes.