I know I'll get pooh-poohed for this, but repair permissions on the hard drive using disk utility in your utilities folder. The last couple of iterations of iTunes had a problem with permissions for some and throwing up errors when it was trying to back the phone up and save files to the hard drive. It happened to me a few weeks back. I did a Google and it said to repair permissions on the hard drive. That fixed it.

Repairing permissions is not really just a placebo.