When your computer starts giving you error messages about how it doesn't have enough memory, DON'T BOOT UP IPHOTO! If there's not enough memory, iPhoto will delete your entire photo library. The photo files, thank god, will still be there, but any modifications you made to the photos in the library or albums or slideshows you may have created will be gone. You'll have to reimport your library folder by painful folder, and if you, oh, happened to have a baby this year like I did, restoring your library will take a long, long time.
Luckily, when this happened to me, I was *only* planning on getting prints made that very day. Prints I've owed people for MONTHS.
I didn't get the prints made. That was the bad news.
The good news was that I actually used this opportunity to back up my photo library, and that's something we should all do. You should do it too. Go buy an external hard drive or find some online service and do it. You'll feel really great afterward knowing that if your hard drive or laptop melts down, you'll have a backup of your important documents.
And if you're expecting prints or a thank you note from me, well...I'm working on it.