Unfortunately, there may be 2 or three different problems that we're talking about in this thread, which could add to the difficulty of finding a solution for any one group. I thought I'd run down the list to see if we have a picture of who this is happening to.
The users this is happening to are running the latest OS and app updates. The problem is that when a user chooses to save a file, Keynote goes through the motions and claims to have successfully saved the file. Some time later, the user tries to open the successfully saved file and is unable to. This is mainly happening to users that are using Keynote's new compressed file format. Some have reported getting some kind of warning (general slowness of the system prior to saving) or have noted that they are at the limit of their disk space, either of which could be a precursor to the problem.
I did a search for SFWordProcessing Plug-in and found this link which I hope is useful for one user.
http://discussions.apple.com/thread.jspa?threadID=1379017
Do we know anything else specific about this issue? The more specifics we have when we report, the more likely it would be able to be repeatable internal to Apple. Has anyone changed their default file format to "Package" to see if the issues go away?