There is a great video how to upgrade your ESX 3.5 host clusters using VUM with 4 phases of video on www.vmware.com site.
1. vCenter server upgrade
2. ESX hosts (cluster) upgrade
3. Virtual machine /tools/hardware upgrades
4. License activation and these phases are straight forward and easy.
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Regards,
Stefan Nguyen
VMware vExpert 2009
iGeek Systems Inc.
VMware, Citrix, Microsoft Consultant