How can I find out who is on a filesystem when I can't umount it?

[Tim Fredrick]

We have a CDROM on our Sun SPARCstation where I constantly have the problem
where umount yields:

  umount /cdrom
  /cdrom: Device busy

How can I find out what process is using that filesystem, and is there a
way to force the umount to happen anyway?  So far I've had to reboot the
system each time.  --Tim