In HA-DRS cluster, the enter maintenance mode task stalls and Virtual Macines are not migrated
VMware Infrastructure 3 Release Notes
In previous releases of VirtualCenter, virtual machines might not be automatically migrated off a host entering maintenance mode if there is not enough failover capacity in an HA-DRS cluster. The enter maintenance mode task stalls at 2% indefinitely, and is not completed even if HA admission control is disabled. This release resolves the issue by allowing the evacuations if HA admission control is disabled. Note that admission control is enabled by default (see Implications of enabling or disabling VMware HA strict admission control when using DRS and VMware DPM (KB 1007006) for more information). Note that the task might also stall if the virtual machines cannot be evacuated for other reasons.