I’m currently working on a system image which is (among other things) supposed to automatically mount CDs as they are inserted. The big problem with this is that I finally had to recognize the fact that with all those hotplug events issued, a media change in an existing device is recognized by the kernel (at least it seems so when looking at drivers/ide/ide-cd), but this is not passed to userland, i.e. the hotplug handler (/sbin/hotplug or whatever you wrote to /proc/sys/kernel/hotplug) doesn’t get an event for this. The only way to recognize a media change that I found is to poll the CD-drive(s) for their media status and handle any changes there appropriately.
Given that the kernel already seems to have a mechanism to recognize media changes, it would really be cool if it could issue hotplug events for them.