Probably not good news I'm afraid. Hopefully you use Time Machine to keep regular backups of your work?
Have you tried to open the file in Graphic Converter (shareware application)?
I find file corruption practically non-existant these days. I would suggest that you maintain your machine. This can be done with Onyx http://www.apple.com/downloads/macosx/system_disk_utilities/onyx.html. Run all the tasks in the "automation" section of this application.
If you can do a drive check - checking your hard drive for physical issues might also be a good idea.