In case anyone is still looking for a solution to this problem, try going to home folder > Library and deleting the "Autosave Information" folder, that should fix the problem.
Thanks! I've been having the exact same problem with TextEdit that the original poster described since about the time I upgraded to Snow Leopard from 10.5.8. After opening a new TextEdit document, I'd start getting these periodic error messages until I saved it manually or closed it. It happened again a few minutes ago, which prompted me to Google the error message, and the first link directed me here.
When I just now checked the Autosave Information folder for my "default" user, gordon, I saw that the folder's permissions were set as:
system: Read & Write
everyone: No Access
And its folder icon had the international do-not-enter symbol on it (even though this user has had administrator privileges for the past several weeks). I just checked my standby user's (gordon2) Autosave Information folder, and its permissions are set to
gordon2: Read& Write
staff: Read only
everyone: Read only
I don't know why these are set differently. I don't recall ever manually fiddling with the permissions on these folders.
I added gordon as an additional permissions class with Read & Write permissions to the problem folder. I then opened the folder, and it was empty. A short time later, the autosaved copy of my TextEdit document appeared in the folder, along with a TextEdit plist file. And I haven't seen another error message.
I believe this started about the time of my upgrade to OS X 10.6 from 10.5.8. Macaroni automatically repairs permissions every week, so I doubt a manual repair would have fixed it.
Thanks again for directing my attention to that folder.
-Gordon