reboot doesn't work.. it was my first thought also.
About running on the edge..i have a 256 GB SSD and i still have around 100GB of free space.
I noticed the problem after deleting a large amount of pictures..around 15GB.
Then i tested the secure empty trash option with 8GB disk images..nothing is working so far..
..I'm close to installing a fresh copy of mountain lion..
There is a thing in iPhoto whereby the first time you delete a photo it asks if you want to delete or just remove from the album, something like that. Unfortunately it is one of these things with a "don't ask me again" checkbox. Now, "deleting" photos only removes them from the album they are in. To actually trash them I must then go to "all photos" and find them, to send them to the trash. To help find them, what I do is rename them zzz or something, so sorting them will get them to the bottom of my collection. Just maybe your photos are still lurking around?