Ok, here is the KB article discussing the problem where the CPU usage of a virtual machine might increase significantly after VMotion migrates the virtual machine in a cluster with DRS enabled. As a result, the performance of the virtual machine might be degraded.
“Starting with ESX Server 3.5 and VirtualCenter 2.5, DRS applies a [...]

Here is a quick fix for those experiencing a problem using VMotion after upgrading from ESX Server 2.5 to ESX Server 3.5
In some cases after this upgrade is done VMotion is disabled and warm migrations are not possible. The solution is to re-enable VMotion on the host.
Re-enable VMotion on the host:

Select the host in VI [...]

If you have a DRS enabled cluster in Virtual Infrastructure you might not want certain virtual machines to ever run on the same ESX host.  They might have certain disk I/O or network behaviors you want separated, or it might just be a case of uptime robustness you want like in the case of multiple [...]