This is more of a quick tip, just in case you start having the same problem that I did.
I made a mistake recently where I pulled out my compact flash card before the iPhoto import was complete. Instead of handling this gracefully, I ended up with a bunch of images in iPhoto where the thumbnails don’t display. Instead, I get a bunch of white boxes with grey dashed-line borders.
Searching the web, I found out that other people who had this problem were able to solve it by rebuilding their iPhoto Library. Unfortunately, Apple changed the super-secret command key combination to do this between iPhoto 4 and iPhoto 5, and there is no help documentation on it.
Still, for those who are curious, here is the answer:
iPhoto 4: When launching iPhoto, hold down the Shift and Option keys.
iPhoto 5 & 6: When launching iPhoto, hold down the Option & Command keys.
Interestingly enough, if you hold down Shift & Option keys at launch, you can tell iPhoto 6 to use a different library folder… pretty nifty if you wanted to maintain different libraries for some reason.
Here is the web page where I finally found the answer. Looks like this guy wrote a clever “iPhoto Extractor” application based on his experiences with this problem.
Psychohistory 
Many thanks for this. Looking up command+option in Google I then found the Apple support page which has this to say about the commands you’ll presented with when you boot up iPhoto 6.
* Select the option(s) for thumbnails if only the thumbnails are appearing in unexpected ways (grey or blank).
* Select the “Rebuild the iPhoto Library database” option if iPhoto unexpectedly quits when launched or does not get past loading photos.
* Select the option to recover orphaned photos if photos appear to be missing from the iPhoto library.
[source: http://support.apple.com/kb/HT2042%5D
I was a bit miffed as to why one would want to rebuild the library and this answered it.