[Update 2/3/06: See this before you try anything!
I am a big fan of iPhoto Library Manager. From my post to the Apple Discussion List:
Apple - Support - Discussions - VERIFY your library prior to update: ...Update 2/2/06: When one imports a Library using IPLM iPhoto 5 creates an system folder structure based first on EXIF tags (if available) and secondly on the image file last modified date. These folders are usually invisible to the user. In my case my iPhoto 5 Libraries, when viewed using the Finder prior to import, had a directory structure that mirrored the iPhoto assigned dates of images. They had this structure even though scanned images lacked EXIF headers. After import, the Finder structure mirrored the last modified date of images. This is somewhat curious. Since iPhoto 6 makes the same change when one updates it's kind of unavoidable anyway.
I am a believer. Please, verify your iPhoto library prior to updating. In fact, verify it every few months. I did this with my iPhoto 5 Libraries prior to updating and I'm so happy I'm buying a SECOND license to IPLM just to say thanks. 
How do you verify? You register iPhoto Library Manager ($20) and you use it to verify your library.
But, you say, IPLM is just used to manage multiple libraries. There are free apps to do that, iPhoto 6 can handle 250,000 images (given enough machine power), iP6 can option-click load separate Libraries anyway.
All right. Except IPLM is the most perversely unmarketed software in existence. The library manager features are free. They are nice, but not essential.
What you get for $20 is the barely mentioned capability to combine (merge, import) Libraries or portions of Libraries (images and albums) with much of the metadata preserved (titles, comments, keywords(!), ratings, roll data, and album membership).
But, you say, I don't need to merge Libraries. I have one Library, my partner(s) and children use the one Library, I don't have a desktop/laptop Library. Ahh, but you do need to verify.
How do you verify? You tell IPLM to create a new Library and import all the images from your existing Library. You won't KEEP the new Library. You do this to test for problems.
If there are no error messages, no glitches, matching image counts, etc -- you're fine. Otherwise, sort this out BEFORE you update.
I have done this with two libraries of about 3000 images each. In each case IPLM/iPhoto identified ONE corrupted JPEG . I was able to restore from old CD archives.
Now I feel confident in the integrity of my iPhoto Libraries. I will be using IPLM to verify them every few months from now on.
Highly recommended. And no, I'm not associated with them in any way.
 In my case I have 3 iP5 Libraries and I want ONE iP6 Library on my iMac -- I decided to do the merge prior to going to iP 6 because IPLM has been used for over a year for iP 5 merges, so it's more tested that way.
 What happens is iPhoto reports it couldn't import an image. On examination in an image editor the JPEG is corrupted. When I went to backups and the original Library I confirmed the corruption. Don't try to open these images in iP 5 -- they will cause iP 5 to hang with a spinning pizza of death. Yes, that's very bad programming. Preview manages them properly. In both cases I was able to get an original good image from old CDs and archives -- but I backup more than 99% of the world. I think these images were corrupted by older/flakier versions of OS X and iPhoto. I have a personal evidence-free belief that the pre-journaled OS X file system was not reliable.