When a CD-ROM is changed in a drive controlled by the cd driver, then the
     act of changing the media will invalidate the disklabel and information
     held within the kernel.  To stop corruption, all accesses to the device
     will be discarded until there are no more open file descriptors referenc-
     ing the device.  During this period, all new open attempts will be re-
     jected.  When no more open file descriptors reference the device, the
     first next open will load a new set of parameters (including disklabel)
     for the drive.

Does this explain why xmcd stays permanently convinced that there's no disk
in my drive after a change, until I quit it and restart?

-s