Just noticed this alert on the VMware support site.
If you are upgrading to vCenter 5, be sure to verify that ALL of your hosts are running at least 4.0 Update 3. Hosts running version 4.0 Update 2 can experience a purple screen.
It is important to note that even though you are only upgrading vCenter, you are still ‘touching’ ALL of your production hosts managed by that vCenter. Once you have upgraded vCenter to a newer release, it will immediately and without warning begin to update the vCenter Agents (vpxa) on all hosts registered to it. Normally, this is a painless non-disruptive process but it’s worth noting that there is at least some level of potential risk involved.
It would be nice to have the option of, prior to the vCenter Agent upgrade, having the system go into maintenance mode, clearing out any running VMs (leveraging vMotion) and preforming the upgrades in a rolling fashion to limit exposure to running workloads.