Today I rebooted one host and brought him back out of maintenance status. This was one hour ago. Until now no VM has been migrated to this host nevertheless we have DRS configured with "fully automated" option and "aggressive" threshold. VC 2.5 & host 3.5
Thnx in advance!
It will only move the vms if there is a performance gain so dependant on how loaded your other hosts are it may not move any until the usage increases.
I would check that vMotion definately works between that host and the others.
Then Disable DRS in the cluster settings and then re enable it and see if anything changes.
Hope this helps,
In ESX4, go to the DRS tab, and click run DRS link. This will force a new calculation, and possibly move vm's on to the empty host.