It didn't happen to me today. But it happened when I was updating my wife's iPhone and then I tried to update mine.
To delete iTunes. Go to the Application Folder and move the iTunes app to Trash and then Download a fresh copy of iTunes from the Apple website.
And then install it.
You should be able to update to the latest firmware.
Whenever this occurs try one of these options:
1. Use a different USB cable
2. Use a different USB Port
3. Restart your computer
4. Reinstall iTunes
If none of these work you need to go the Apple store and they will figure what might be the problem.
Hope this helps