Device is busy

If I get an error saying that a device is busy (like /dev/dvd), how can I tell what 'has' the device and how can I release it in order to unmount it? -- Cheers, Trey --- "There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another which states that this has already happened." --Douglas Adams