I had been working on a Blender file for about an hour when my whole system just crashed and powered itself off (using Ubuntu 13.10, and I had many applications open).
The /tmp directory where the autosave file is typically found had no blender files in it.
I just opened Blender again and confirmed that the autosave feature is working properly according to my settings (saving 2734.blend every 3 minutes).
So why is there no autosave file? The /tmp directory had other files in it, so I don't think that Ubuntu crashing caused the file to be deleted.