Sorry for the late reply. Don't always see mount: / busy . However It still is not cleanly unmounting the filesystems, whether I use the Power off System or Reboot from the menu or give a command such as sudo poweroff or sudo halt. I don't get it shutdown on a PC running linux will kill all processe...

After I got Raspbmc set up like I wanted I started getting mount: / is busy when shutting down.
I thought the problem may have solved it self, but even when I don't see it if check the sdcard in my
other computer the filesystems have been left unclean and I have to run fsck.