Following 2.1 update this error came up when syncing with Itunes.

Solution was simply to restart the computer!!!!

Other people apparently had to do this:

If you have sync issues (error 13213) with 2.1 do the following:

1. Unplug your iPhone and turn it off. Hold the power button at the top until the red slider appears and power it down completely.
2. Quit iTunes 8
3. Relaunch iTunes 8
4. Plug in iPhone - DO NOT TURN IT ON FIRST. Make sure it's still off completely from step 1.
5. Should sync normally