Users of Citrix XenServer 5.6 are experiencing a problem where the cdrommon process begins consuming all CPU resources and affecting virtual machine performance. Using 'top' or 'ps auwx' from dom0 shows that the process is consuming 99% or more CPU resources. Users of Xen Cloud Platform (XCP) 0.5 have reported a similar issue.
Killing the cdrommon process fixes the problem until the next reboot. It has a nasty side effect of making virtual machine migration impossible. If you aren't using a pool, this may not be an issue for you.
Although I personally haven't seen reports of this working, the 'xe-toolstack-restart' might fix this issue when it's occurring. This would obviously only be a temporary workaround.
To date, Citrix has not released a fix for this problem. Keep an eye on the XenServer 5.6 updates page for future hotfixes that address this issue.