Why does my iTunes say my backup is corrupt?

I was looking through iTunes today and became puzzled when I noticed it said "Your iPhone has never been backed up to this computer". I know I've backed up my phone quite a few times in the past.

Oh well, I thought I better get a backup going. Clicked on 'back up now' and after a moment, got the message "iTunes could not back up the iPhone because the backup was corrupt. Delete the backup for this phone in devices preferences, then try again."

Firstly, this phone supposedly has never been backed-up to this computer before. So what corrupt backup are they referring to?

Secondly, in device preferences the backup field is empty. So I have nothing to delete.

Any suggestions? Please don't suggest reinstalling iTunes because I just did that just now and I'm not looking forward to spending a few hours of my Sunday doing that again.

[Re-Titled by Moderator]

Original Title: Cannot back up because the backup was corrupt ?