Kevin,
I looked at the file you uploaded and it is definitely corrupted, but in a way that Photo Mechanic just wouldn't do unless your computer is badly misbehaving (after all PM is used to caption hundreds of thousands of photos daily without any problems). I noticed you are using Mac OS 10.5.4 now. Have you seen this problem in the past? What is the file system of the disk you are editing your photos on (e.g. internal vs external HD using FireWire, MacOS vs FAT, etc). We know that Apple made some significant changes in 10.5.3 to file I/O that apparently affects FAT file systems (e.g. flash cards), and 10.5.4 had some other changes that also affect file I/O. So my first thought would be that there is some file system corruption or possibly even bad memory (RAM). Can you run some diagnostics?
--dennis