I have the exact same problem you're having. I was unable to figure out too.
My guess is that a service that is currently running exsists in a location on the drive that the Boot Camp partitioner wants to move. (Since the service is running, and probably has a lock on the file, the partitioner can't move it.)
I might try a reinstall of the OS to get the machine back to a factory-install state, and see if that fixes the problem. I don't know when I'll get around to doing though. Maybe you could give it a shot. I also need to make sure that absolutely NOTHING extra is running when the boot camp wizard is run. (Things in the tool bar for example, like MenuMeters, or Gmail Notifier, or Adium, etc...)