Do you know if you have a Mac with an NVIDIA based card? There is a large thread, +not directly related to iPhoto+, about NVIDIA drivers freezing systems:
http://discussions.apple.com/thread.jspa?threadID=1916253
Myself and at least one other person have seen this happen even more frequently with iPhoto '11.
You won't see the error message in the Console All Message log. To see it, you open Console, expand FILES, expand /private/var/logs/, and look at kernel.log.
If around the time of the system freeze and you see errors like:
*NVDA(OpenGL): Channel timeout!*
*NVDA(OpenGL): Channel exception! exception type = 0x6 = Fifo: Parse Error*
Then you are likely having a driver issue that has gone on for a while with NVIDIA based Macs.
Not much you can do. Hopefully it just freezes the system for a short time and doesn't require a restart.
-Kevin