If the esx host is part of a drs cluster in fully automated mode vc will vmtoion all the running vms off of the host to the other nodes in the cluster when you place the esx host in maintenance mode - if you are not running drs or drs is in partially automated/manual you will need to vmotion them manually-
Maintenance mode should do it for you with limitations listed above.
Message was edited by: gary1012
To automatically move them you would need DRS set to fully automatic. If you're interested in Powershell you could all vmotion all VMs from one host to another (see page 13 for an example http://blogs.vmware.com/vipowershell/files/vmworld_europe_2008_powershell_lab_7_manual.pdf).